The big ticket items that you have to pay for are airfare, hotel, car rental if you're not within walking distance of the convention center, and meals. If you can use points for airfare and have a friend in Houston, that will help.
Shopping the vendors is optional, so you can control your costs there. Same for any classes.
I went last October with a friend and we spent 5 days, which was the perfect amount of time. The convention center is HUGE, and seeing so many quilts at once was overwhelming. We'd go look at the quilts in the morning, then go back to our hotel (2 blocks away) for lunch, then sit by the pool for an hour or so, then go back to the show. We wandered around downtown and tried a few restaurants, found a bookstore, returned to the show, etc. It was amazing how many quilts we saw on the third or fourth day that neither one of us remembered seeing before. It was nice to have 5 days, we really felt we got to see and study all the quilts.