Wow, that's super cool. I don't know what your cost in materials and labor is, but if you can make a profit at about half of what you are charging, I think you will have as many orders as you can handle. I'm afraid $250 for a shipped custom key cap set is going to be a tough sell.
Then again, I see people paying well over $100 for GMK ABS sets, so you never know.
I've been spoiled by IBM's fantastic PBT caps, so I would like to get MX PBT sets someday.