"...and you know this how, exactly?"
The creator of the game checks the Xbox Live leaderboard to see how many copies have sold since you are automatically put into the leaderboard when you buy the game. A week after release, he said 55,000 copies were sold based on the number of entries. When I checked in September, that number was at 150,000. Since the game sells for about $15 in Microsoft points, that's about $2,250,000. I decided to check again just before posting this, and that number has jumped to 211,749 or $3,176,235. I'm sure Microsoft takes a hefty cut, but that's still a very impressive showing for an indie game with no budget to speak of.
Capcom's last Mega Man game also sold well. It's obviously not an indie game, but it runs on an emulated NES engine so it's in the same spirit as games like Braid or Cave Story. Capcom also did a remake of Super Street Fighter 2: Turbo with high definition sprites and it has been the best-selling game on XBLA since it came out in late November. They're definitely ahead of the curve as far as mainstream publishers are concerned.
People are starving for cheap side-scrolling games with clean 2D graphics and retro gameplay. The key word being 'cheap'. A $15 or $20 game is an impulse purchase. $60 games are only going to be less attractive as the dollar plummets. History has shown that entertainment sells during times of economic hardship, but you aren't going to sell anything if you don't price it right.