It’s $149 in the Mac App Store and it is the RAW photo editor that I use. Aperture is nice, but Lightroom is great. I have only been on version 4 for a little while, but it is a nice upgrade. Lightroom is still the absolute best tool to use for reducing noise in your images — hands down.
One important note (that I didn’t know) is that buying the Mac App Store version only grants you a license to use it on a Mac, whereas buying the downloadable version from Adobe (same price) grants you a license to use it on Mac or Windows. So if you like to use Macs and have to use Windows, this is something to think about.
Update: Jim Dalrymple notes some more caveats with Lightroom 4 in the Mac App Store — so many I would recommend that you buy it from Adobe’s website instead.