I used to have an ad unit that used to get a CPC close to 1$ (tier 1 traffic). For testing purposes, I removed that ad unit for about 3 months. Now when I reintroduced the same unit exactly at the same slot (think of a website layout-restore as it was three months before) the CPC is about a fourth of earlier levels. Is this normal? Does Google store and use ad unit-level data in estimating which ads to serve? If so, will the system aggregate data before serving the usual mix of ads/ad categories? Thus gradually helping the CPC catch up to the earlier levels?