Why is the price so high? Stoneage gamer has them for $140??

Stoneage has a 3 THREE year warranty on their products For the price amazon is selling!!!!! Crooked Amazon probably charges them more so these 3rd party sellers have to raise their price.

Does it come with roms installed if not where and how do i get them?

No, because that would cause alot of legal issues. However there are tutorials on how to get the roms on youtube and install them to the cart.

What does this do? Is there games on it?

Basically, you can consider one as an adapter with an SD card slot. You can download games from the internet, copy them into an SD card and play on an original hardware. The main point - this is not an emulation, hames will look and feel the same as on original cartridges. It's a very convenient way to have all games on one single cartridge.

What are the differences in the x3, x5, and x7 models of the mega everdrive?

The main difference is - X3 doesn't have memory expansion for a Sega CD. X7 is much faster and comes in a fancy case. In addition, X7 has a lot of features for a homebrew development. So, X5 - is a strong middle. If you have Sega CD, I'd suggest you, to go with a X5!