An easy way to figure this out is like this...take the total number of horses you pick in each race and just multiply them all together - then take that number and multiply by $2 (or $1 if doing a $1 bet) and there you have your total cost.
So if you do a Pick 3 bet for example and want 2 horses in Race 1, 3 horses in Race 2, and only 1 horse in Race 3 --- 2*3*1 = 6. Take 6 and multiply by the bet amount of $2. Total bet = $12.