I'm sorry to hear of your pending job loss. As for doing TopCoder as a full time job, here are my thoughts on the matter. Yes, there are a few here that do TopCoder full time, and I thought about doing it full time on many occasions. The issue is that we are all "sharp coders", and a "decent living" is relative.
In order to get paid, you have to actually win the competitions. You may put in a lot of hours into something, get 3rd place, and have no prize money to show for it. Also, competitions come in spurts. Sometimes there are a lot, and then there are times when there are hardly any. You may win first place in development competition, collect your $400, and that would be the only competition for the month. Now, $400 a month may be enough to live off of in some countries, and no where enough in others.
Yes, you can make money doing TopCoder. Yes, it is possible to make a living off of TopCoder. But before making up your mind, try TopCoder out. Get up to speed by doing a development competition or two. Worst case, you have some extra experience and skills to add to your resume when you are job searching.