Frequency to voltage converters
Use a CD4046 PLL at 1:1 frequency ratio.  The VCO is reasonably linear (1%), so its control voltage will be proportional to the input frequency.

Looks like CD4046 sells for $0.15 - $0.20 in quantity:
http://octopart.com/partsearch#search/requestData&amp;q=cd4046

Digikey sells them for $0.354 ea in 100 quantity (DIP, other packages cheaper):
http://www.digikey.com/product-detail/en/CD4046BE/296-2052-5-ND/