i dont know pricing, but i can tell you more or less how they work
intercoolers work by atmospheric temperature air leaching heat off the compressed air
its to do with physics etc
keeping it simple though, the air rushing through the intercooler cools down the charge.
water to air intercoolers have a section of piping surrounded by a water jacket that does the same basic job as a standard intercooler
except that water is a much more effecting substance at removing heat from the air
one problem you can get though, is heat sink, in which case everything messes itself.