Sounds like you are suffering from a couple problems.

1) 60 websites. Invest your time into a few good websites, get rid of the others.
2) The RPMs will be different for each site. It all depends on the quality of your traffic and the location. If 1 site gets 99% of it's visitors from india, it'll earn pennies compared to the one that gets 70% of it's traffic from the US.
3) What do you mean by 'expensive and time consuming to effectively market'? One thing to keep in mind, paying for things like facebook ads, adsense ads, etc results in lower quality traffic, which drives down RPM. AdSense works best on sites that thrive on organic search traffic. Paying for traffic is typically considered a no-no. Any paid marketing efforts should be one time shots that boost traffic permanently. If you have to pay to drive traffic to your site, you'll always have an uphill battle to stay profitable.

My sites are focused on localized topics, like news and weather. Each site focuses on a different area in the United States, but all within a fair proximity of one another. So 99% of the traffic for each site will come from people that live within that specific region.

(We actually block most non-US IP addresses at the firewall, since we do not target any traffic outside of the US.)

The one site is doing great, but the potential traffic for the site is limited by local population; we've reached a 100% penetration in that region, so we'll never have more traffic than we have now. This is why we added new sites; we cover more cities, and we increase our potential. The problem, though, is that the new sites have a very low RPM as compared to the larger site.

Obviously, my employees work to maintain the larger site, as that is the majority of our income. But as the company owner, I'm trying to find ways to make the other sites grow, too... with the hopes that at least some of them will become as large as the original.... or, hopefully, larger.

By marketing, though, you're 100% correct about paying for traffic, but it's a catch-22; if no one knows that a site like mine exists for their region, then they don't search for it, and will never find it. So we do pay for Google ads (with fair results) and Facebook ads (with terrible results, if any), targeting each specific region, but in the hopes that the people that click on the ads will return to the site regularly.

That can be ridiculously expensive, though. At an average of $2 /click, we pay $3,000 to reach 1,500 new users. If every single one of them returned to the site every day and looked at 8 pages a day (which is about a normal rate for us), and an average RPM of maybe $1.50, it would take about 6 months for us to break even on the campaign! But all 1,500 don't return daily, so that's obviously a losing strategy. Worse, some areas are in higher demand, so that PPC can be up to $10!

We also incorporate things like billboards, giving out promo items, sponsoring local events, etc, but that's very time consuming. Covering 60 regions means that we could spend a maximum of 6 days in each region per year. Currently, we're doing exactly like you suggested (focusing on 3 at a time), but at the rate we're going, by my count it will be about 20 years before they're all effectively marketed.

Technically speaking, all of these sites are really a single site, and the domain acts as a mask; eg, www.keyword.com is a mask for www.lousydomain.com/keyword . So in theory, I could redirect every user to lousydomain.com/keyword and possibly double the RPM, but then I lose the SE-friendly and memorable domain, etc. Worse, I haven't been able to find a good umbrella domain; the ones I have simply aren't that memorable and do not test well.

So that brings me back to the original question. Other than consolidating everything in to a single site, can you think of any other way to maximize the RPM on them individually?

How much time are you giving them? I have similar issues with (let's say, more than 50) websites that deal with various seasonal events targeted to specific states. I didn't expect any of them to overtake my flagship site (which was created in 1999 and still grows organic traffic every year) yet this year, two of my other sites overtook it - and those sites are four years old. Other than that, I don't bother comparing them to each other, but I set goals for them, and I allow at least two years for a site to really 'ripen' into profitability (mostly because it takes that long for sufficient word to get out)

You and I seem to have quite a bit in common, then, netmeg! :-D

My oldest site began in 2001. The second oldest began in 2004, then #3 and #4 began in 2006. The newest was launched this year.

They're all growing exponentially, so that's not really the concern (although none of them are even close to the flagship). It's been a time consuming and expensive process, sure, but that's not really the issue.

The issue that prompted me to make this thread was the realization that 7 million apples in one barrel are worth x amount of money to Adsense, while 7 million apples spread out in 59 buckets are worth 0.2x (roughly 20%, maybe less). It's odd; the quality and demographic of the traffic is virtually the same, and the site designs are similar, but because they're on different domains they're worth considerably less.

Based on this, I'm trying to find a way to increase the value of those 59 buckets. The obvious solution is to increase the number of apples in each bucket, but as discussed, that's not a simple or cheap solution. And presumably, dumping them all in to a single barrel would make them worth more, but then I lose a lot of value myself. So I'm hoping that there's a 3rd solution that I've overlooked.

60 sites make my head spin.

I don't have a solution for you. My solution is just time. Some of my sites will probably never make a ton of money, and others are little seasonal gold mines. But all the markets are different, so there's no way they are all going to perform the same. I also don't spend anywhere near what you do; no money is spent on traffic; what I do spend is on development and things like hosting. I spend my TIME on making them easy to use and popular, so that people will come back over and over, and I think that's why the traffic grows every year. And with the traffic, comes the AdSense.

60 sites make my head spin.

Mine too, and it wasn't what I set out to do originally. Some are definitely put on the back burner in favor of the ones that perform, and I'm sure that has cost me over time. But as long as they are making a little more than they cost me, I leave them sit till I get around to them.