Don't be so tragic, man. Think of it as going to a bar to see your favorite band - there's a cover charge AND a drink minimum on top of the show ticket in many bars. I know I went to a number of shows that way and drank the nastiest, cheapest beer I could because my finances were tight... but it was worth it. Did that to see Warren Zevon in a little club; I was about ten feet away from the dude. Worth every penny, despite the fact that I quite literally missed meals over the following week-plus.
You just have to ask yourself if it's worth it to you. Who knows? You might even have fun at the con itself!
The difference being that at least you got a show. The way I imagine it is that I stand in line to get a day pass. Pay $20, go to the signing, wait in another line. Get to talk to Jim for about a second before I'm ushered away. So at least in your situation, you got to hear them play. (Am I right?)
So basically I get to pay $20 to stand in two lines, plus annoy my parents to take me to Richardson (since I don't have a car) for an autograph and maybe a second of talking to Jim? As much as I like his writing, I can't honestly say it's worth it. And if they have to cancel for some unforeseen reason? Well, I guess I'm screwed. Chances are that they wouldn't give me a refund because there were be other things to do there, sure, but then to me the $20 and time would be wasted. (I am a bit of a pessimist, but I see this as a way of being realistic.)
I wouldn't mind going to the convention, but I'm not sure I would have time for it. Besides, there's not really much there that I'm interested in. I've heard of two or three other authors off that list and have read none of them. I don't relish the thought of paying the money for this, but if I could go and hang out with some of the board members before or after meeting Jim, it would be a little more worth it.
If I could be assured that Jim is going to come back for another signing (like say somewhere that doesn't cost any money...don't the bookstores let you get a signature for free?) then I would just skip this, but in the past, I've seen author's itinerary for tours and most don't say Texas on them and even when they do, they're usually in Houston. So, yeah...I am a bit upset about it.