Razie, I wish we could get $20 grinds!!!
And that was $CDN, so like even less
they did a good job too: Corbetts in Oakville. Had 15 skis done beginning of last season...
I see Middleburry's list in James's post there also quotes "factory refinish" at 20$ I would assume that's the basic base grind?
If you're really good with a belt sander, and the skis aren't all that expensive,
nobody is good enough to belt sand a ski flat with a portable belt sander. Not that I haven't thought about it...
If you really do want to sand a ski, then...
Start with 150 and then 220 grit Aluminum Oxide sand paper. Do not use "garnet" sand paper (typical for furniture) - it will scar the base, as it loses "grits".
Wrap the sandpaper on a flat file. I use a round 1 inch hardwood dowel - which you can roll slightly as you move along the base, exposing new sandpaper without stopping to rewind. Try to keep pressure constant and even on both edges. Don't press directly on the edges, as it may result in a convex ski base - that's a technique to get the proper base bevel with a file, free hand, but I would not attempt it if you haven't done tens of skis...
Follow up with a steel brush and fibertex pads, to smooth out the grooves in the base.
Alternatively, use the Ski Visions base flattener tool or a stiff steel scraper, carefully - it can speed up the cleanup job.
The trouble with sanding the base is it requires quite a few tools to clean it up properly.
Then do the edges, with diamonds in the proper guide (0.5 or 1- the sanding will leave them a little less flat, you'll see) and then file/diamonds on the side to restore the edge.
It can be done. It's not a big deal, but it takes a while though and you have to be careful.
More details
here.
Good luck.