Best charges a fortune for 1200XL keyboards ($45, I think).
There is a thread -- check out #6. At the end, someone recommends simply cleaning the mylar contacts with a Q-tip dipped in alcohol. I decided to try this. After removing all those tiny screws at the back, I very carefully peeled away the mylar to the halfway point (lengthwise) and cleaned each mylar contact (especially the ones that form a link to the ribbon cable). Then I pressed the mylar back in place and did the same with the other half (this avoids pulling the whole thing off and then having to realign the sucker). When it comes time to put the screws back, I recommend starting with the two screws around the main contacts, then going to the four corners, then doing the rest. Don't over-tighten!
This has recently worked for me on a 1200XL keyboard and two A800 Mitsumi keyboards (same construction).