If I discard the 2d, the odds are that I would get back an average of $2.15 for the 47 possible draw cards. If I keep the straight, then I get a guaranteed $4.00.

If it were an open-ended straight flush draw, say 9h-10h-Jh-Qh-Kd, you would only get back an average of $3.43 for the 47 possible draw cards, still not a good bet.