One, shouldn't this be in the Child of Eden forum?
Two, I played and finished it last night and it will definitely take you about 90 minutes if you don't die (you will, probably), but it's not about "beating" those five areas and never playing it again. There's a challenge mode in a new area that unlocks after you beat the game on normal, a hard difficulty, tons of stuff to collect, and if you're into achievements, then you will be spending an unholy amount of time getting them all because they are hard. You absolutely have to play through it once to even get good enough at it to bother with anything else the game has to offer. It wasn't until the last area that I finally felt really confident and solid at the game. Now I just want to play it again.
Anyway, may not sound like your thing, but even if I never played it again, my $50 was in no way wasted. It's incredible. I would buy this game at full price once a week for the rest of my life if it meant I could go back in time and not pre-order Homefront, for example. How long a game lasts should be one of the last things you concern yourself with. Its quality is all that matters, and Child of Eden is of major quality and endlessly replayable.
We forget how expensive NES games were ($40-$50 in the 80s) and how many of those you could blaze through in half an hour.
I see nothing wrong with this. I did it recently with the Magicka DLC. Haven't finished Magicka yet, so who knows if/when I'll play the DLC, but it's was a buck, a donation to help Japan, and a TF2 hat for me. Rule.
If it's an independent developer, you're giving them money to keep going, getting a hat, and possibly getting a game you might play and enjoy someday. No big deal.
Now, if you spent $50 on Red Faction Armageddon for the sole purpose of getting the Maul for the Pyro, then we've got a problem. Or we don't. Spend your money on what you like. If that's TF2 hats via Steam games you'll never play, awesome!