There was of course no set percentage or dollar amount, you'd have to research that for the particular model and era you're interested in.....
For a rough idea of what a B30-7A likely were priced at, a quick search of Trains locates a quote from EMD of $719,600 for a GP38-2 from EMD in the November 1982 issue of Trains from that same timeframe. So we're likely looking at a savings of $50,000 for a locomotive priced at roughly $750,000 with a cab.....
And we're of course just talking purchase price as well. Another savings the railroads always would've factored in when estimating what they'd save by buying a cabless locomotive would've been the savings in cab maintenance for the life of the locomotive.