It is kinda high, but you have to think about it though: Most games are developed for a few years, sold, and then they start work on a new one to sell for more money. This one isn't a short term project, its something the developer plans to work on his entire life. The first time I saw this game was over 10 years ago when it was more fantasy themed with dungeon delving and gold coinage. It has changed and evolved constantly, the different versions I've played over the years have been so vastly different that it's more like a whole game series than a single game. You're literally buying a lifetime registration for it, for every update throughout the developer's lifetime.
If you don't want to spend the $55 just buy the $3 version, its honestly probably the best option for most people - you'd have to buy 19 minor versions before it would have been more economical to buy the lifetime. Even if you bought one every 6 months, that would be nine and a half years. The $3 version buys the big bug fixes too, it just stops after that version gets stable and the next version rolls around. It's not like you're getting half a game, you have a fully playable game.
Paying $3 here and there every year or so when you decide to play it is hardly expensive.
Honestly he would probably be making more money with DF's model of giving it free and accepting donations. Most people buy the $3 version and thats it, and even the people who shell out the $55 aren't giving THAT much money over the long term. DF's donation model has lots of people giving $10 here and $5 there and over the years I'm sure many of the donators have given more than $55. Just think of the $3 as a small donation for his work on the most recent version, the same way you pass Toady a few bucks when he comes out with a big release.