hi
As a guide one could use the place dividend to find the value for 2nd and 3rd.

$4.00 the place approx. or above. With trifectas you must find value as long runs of outs can cost you plenty up front.

I rather miss the small ones and get the bigger dividends there is a formula you can use that will give you the approximate return for a dollar for your trifecta combination.
x(xy-1)(z+1)
______________
x+1

Lets say we have three horses say 7/1 15/1 60/1 lets assume the 7/1 wins the dividend will be different if the 15/1 comes second to the 60/1 coming second.

This is because alot of punters dont put 60/1 horses for second. So the pool is the same say $30000 if 50 punters get it with the 15/1 coming second then the dividend is $600 but if the 60/1 comes second and only 10 punters get it then the dividend is $3000.

If you can find the value for second thats the key.

regards
ubetido