Do you care what airline?
Which NY airport to you prefer to use?
How many days do you want to stay?
What time of day is acceptable/unacceptable?

Depending on how flexible you are on dates and how long you want to stay, it can be had for ~$230-$250 per ticket.

For instance:
$229.40 on American, out of LGA on March 17th, but you have to leave at 9:05 PM and arrive MIA at 12:05 AM.
That's the absolute cheapest I see, but there's a good bit of flexibility in the $250 range (and out of EWR or JFK).

I wouldn't bother with using miles for that, but if you want to, and you don't mind travelling on Tuesdays and Wednesdays, you could probably have it for 25,000 miles on Continental (or others, but I'm quite certain you can do it on them, and they're an AMEX transfer partner). Of course, if you're willing to transfer to CO and accept $.01 per mile, you might as well use pay-with-points through AMEX and earn miles on the flight. I wouldn't do either, I'd just pay for it for that amount, but it's certainly an option.