You can buy up to 50,000 points per year. They cost $12.50 per 1000. The 15,000 points would cost you $187.50.
It's just my opinion, but I've never found purchasing points to be a very cost effective option. 15,000 points is a 1 night stay at a Catagory 3 property. I've found that it's more cost effective to pay the nightly hotel rate rather than to purchase points to use for hotel stays. When you compare the $187.50 cost for purchasing 15,000 points to the hotel nightly stay rate, I'm willing to bet that you would be money ahead to pay the nightly hotel rate.