Have to say I disagree with you here Luv2bet. If you take an exacta on a short priced horse as first and longer priced horse as second you shouldn't expect to get as big a divvy as if you take the longer priced horse as first and the short priced horse as second. Obviously the chances of the shorter priced horse winning are better than the longer priced horse winning so the exacta divvy will be less.
Following on from this the quinella should be the average of the two possible exacta prices so if the shorter priced of the exacta's came up there is no reason why it should be double the price of the quinella.

I find if I bet on exactas using my best and second best rated horse in the right order I actually do better overall (only by about 5%) than if I put the same amount of money on quinellas using the same horses.

While I'm being disagreeable :-) I'll disagree with Angel as well. If I'm sure of first and second I'll take an exacta I think is going to win rather than a trifecta that, although it might be worth more, is probably going to loose.
My system comes up with 8.00% strike rate on exactas, and only 2.23% on Trifectas - 1 win in almost 50 is a long time between drinks.

KV