Not sure if it's the cheapest......
165cc emission or non-emission $1275/pair
185cc emission or non-emission $1275/pair
205cc SBF race head $1675/pair
225cc SBF race head $1995/pair
Spring/valve upgrade $135
Machine heads for O-ring gasket $100/pair
http://www.houstonperformance.com/
I got mine from there, it's a smaller shop, but Steve (owner) has given some good deals.