That's not how steam works. When you purchase something through steam, you can download and install it as many times as you want. If your computer crashes, all you have to do is reinstall steam log into your account and redownload it. This is one of the reasons why steam is so popular.
as far as the price. $50 for games is standard, especially for pc ( and I guess now mac ). It costs millions of dollars to make and promote a game. Most games for consoles are $60 and there PC counterpart are $50 because both microsoft and sony take $10 off each sale. If you are concerned with the length of the game, go read a few reviews and they will all tell you the game is much much longer then the original, plus it includes a co-op mode with different levels. $50 seems pretty good to me.
Also, it's supply/demand. Millions of people are going to go out and spend $50 on the game, so why not sell it at $50? The nice thing about steam is, if you can find it cheaper elsewhere, you can still install it on your PC/Mac and play it through steam. But a lot of people are willing to pay the $5 extra to get the convenience of not needing to ship or wait in a line to buy the game.