It is very difficult to estimate these things. I believe that last question cost you a point, but it wouldn't make the difference between a Q44 and a Q50. The effect this had on your verbal score is impossible to quantify in any meaningful manner.

I am accustomed to going by the GMATPREPs - they are the best and most accurate predictor of your final score out of all simulation tests out there, and even then there's a 20-30 point margin either way attributed to pure, dumb luck. So I guess my reply to your query will depend on two things:

1) What were your GMATPREP scores, the first time you did the two CATS? (scores on retakes of the GMATPREPs are less indicative, since some questions will repeat). How far are they from that 640? If this score is indeed a 90 score drop form your usual par, I'd retake. But if that 730 was on a non-indicative test, then, sadly, i'm not sure it's relevant.

2) Based on your profile, will that 640 put you in the running for Michigan? If so, I'd put ego aside, save myself the $250 of a retake, and spend the time on making a killer application. Consult the many application experts on this forum for advice.

Whatever you do, good luck!