Tricky subject.
First, if you're exchanging diamonds for blowjobs you are in too deep. You need to be exchanging the prospect of diamonds.
Second, I would never pay more than $50 for a bj. Never. Over that limit you could take a 'date' to a nice restaraunt and get her to do it. So, your math should be total price/$50= bj:carats.
Third, the diamond exchange rate can be gamed. If you must give diamonds, give used diamonds. You can ALWAYS use the dead aunt heirloom story. Used diamonds are about 1/2 price, but women have no clue what they are doing (ever) so you can get full bj:diamond exchange for them.