The Reason why I need to use DFP is because I receive many advertise request every month. I don't want to lose these opportunities to maximize my revenue, however if I see 50% down adsense revenue, I would require to hardcode adsense code again every pages and it is not practical to change ad codes whenever I find new advertisers.
My other question is after using DFP my revenue dropped as below
US rpm dropped $16 to $9 - UK $20 to $5 - india $3 to $1

I know GEO targeting is possible. is it possible to set different rate for each geo location for the same ad? let's say US visitor must be greater than $15 and indian visitor must be greater than $3. Thank you.

Check your contextual ads RPM, this might be the cause

You might want to check your ad sizes - RPM changed a lot in the last few years. If you use dynamic sizes (DFP positions with multiple sizes such as 300x250, 336x280, 300x600) you have to keep in mind your ads will fill any of the available sizes however AdSense fallback will only use the first size in the list - and seeing prices changed in the last few years you might have to think which size order you put in your code.

I checked contextual ads RPM is also fine many impression and click I have only 2 ad size for adsense and I only use these 2 into dfp I did increase $1.30 and now I am reducing $0.70 if I don't see any better I will revert back to adsense code again.

If your raw-adsense rate is $9-$12, you will need to do either of this:

1. (by default dynamic allocation is usually on, and you might have missed this) you need to traffic dfp ad slots at that price point as well at $9~$12 minimum, if you have ad slots at $1, it means that DFP will fill those with $1 RPM "First", before trafficking the rest of the impressions to your raw adsense. Which means that you are giving up your $9+ RPM rate for $1, which makes sense with the numbers that you are seeing.
2. Turn off dynamic allocation.

As a side but important note, if you are not using any other networks, raw adsense code might be the easier (better) implementation. A pure DFP to raw adsense implementation will suffer certain degrees of decay through server hops.

Geo targeting with different CPM is possible, but you will have to play with either prioritization or frequency cap. It takes a bit of experimentation.

Thank you very much your answer I just checked my adsense coverage was 100% before implement dfp and now I Shocked coverage dropped to 60% after 10th of feb . Ofcourse if there again adsense didn't do anything stupid .

Oh I can see why my 40% viewer use internet explorer and IE shows blank ads why I didn't check with another browser didn't think there was going to be issue silly me ! now I need to find out why DFP is not showing on internet explorer makes coverage 60%