I can't comment on cost, but I'm pretty sure you would need to re-fit the headstock, carriage, and tailstock after grinding the ways.
If the grinding shop knows its business, scraping may not be necessary. Scraping might be the easiest way to re-fit the headstock, carriage, and tailstock though.
Or, in theory at least you could forgo the grinding and do it all by scraping, but it would be a heck of a lot of work to scrape all the lathe ways to-form and parallel for 36 inches each.
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