I have several Arcade1Up cabinets. I can assure you that they work, and are a suitable way to get a curated set of emulated games. It's plug-and-play at its finest, and there's ample sites online that will give you instructions on how to hijack them for broader emulation use, if you are so inclined.
Quarthios is correct that most of their cabinets (including the one linked) are about 3/4 the size of the original arcade cabinets, and are better played sitting down, or placed on a high table without the riser. However, they have recently (within the last year or so) started making what are called "XL" cabinets that are full-sized.
Here's the Pac-Man XL. Note that the Pac-Man XL measures at 66" tall, while the "Legacy" ones are 55-58" tall.
Edit: As for the original question: $300 is a good deal, all things considered. Buying a DIY cabinet kit from a shop usually costs around $300, and doesn't include anything but the frame. No monitor, no emulator machine, no games. And that might not even be a full-sized cabinet, it might just be a bar-top model.
Second edit: I think one of the things that struck me the most about the A1Up thing is the number of games on the consoles. Not that it's too little, like many emulation fans cry - it's that it's
just right. For most people, once you get above ~100 games, you start to push into analysis paralysis territory. The big-time emulation packages (Pandora's Key, etc.) having thousands of games just makes it more likely that you'll either play one of a very short list of games you really know and like... or not play anything at all.