The price was from JerboaJohn's post about nrracing.com having the engine, but now that I have actually checked, they are just listing the GXH50 at $359. It says 5/8" shaft, folks, that's going to be the key difference between GXH50 and GX50! I still haven't actually seen a price or the engine for sale yet, we're all anxious!
Darwin, you're right about the GX being considerably more compact when bolted to a rackmount, because the GXH has that big base on it and the GXH50, having the 5'8" keyed shaft, actually needs some kind of adapter bolted to it for the 76MM mounts. The tapered-shaft HS49 and HS53 don't need that adapter, but they are considerably wider, especially the HS53 with it's thicker fan housing for it's 12v genny. They do hang out far. The large mounting base at the bottom of those engines makes them appear much larger than the GX35, but they aren't really that bad. I do think the GXH-style looks goofy on a rackmount tho LOL
I'm betting this new GX will have a better powerband, slightly more compact footprint, and weigh a lil bit less than GXH.