Are they good? I have no idea, because I've never used them.
How much does it cost? As much as you paid for it, I guess...
How do they compare to MX? Again I don't know, but maybe close to reds?
It seems like the keycaps might be compatible with MX caps, but I'd have to see the underside of one to know.
It most likely started life as a beige color, which has yellowed over time. If you wanted to try and restore it, do a search for retrobrite.