According to Wong's book, fezzik and jack48 are correct. Wong has compiled a table based upon how frequently a spread has taken place in the NFL from 1985-2000.
According to his table, a spread of 3 takes place 10% of the time. Therefore, paying 20 cents or less for a push or win on 3 makes sense. A spread of 7, 14, and 17 occurs 6% of the time for each spread. Paying 12 cents or less for a push or win on these numbers makes sense.
All other spreads occur less than 5% each so buying into those numbers for a dime is not worth it.