It depends on the contest. Some are worth little, such as the ones Wagerweb runs, the one where you pick a 3-team parlay ($25) isn't very good, even Hollywoods $10,000 weekly one has too many entries.
Rx has good ones run by RIO/ Sky. SBR has a good one. FBZ has some good ones.
I figure the odds of hitting it vs. the time factor. Some good ones here, as well.