Prices do rise on performance ad networks in December because more brand advertisers advertise. Brand advertisers tend to bid more in the auction and tend to have better content which causes prices to rise. I looked at Jumptap's Oct '11 - Feb '12 data. The lowest CPC for this time period was October. Nov. saw a $0.026 rise in CPC. Dec. saw a half cent rise from Nov. Jan. saw a drop of $0.024 in CPC. Feb went up slightly from Jan.
Hope this helps.
Hope this helps.