Probably not that much more. Actually you'd probably get less as the shop will try to profit off the fish and give you only a portion of it's value to them.
Has your plec outgrown its tank or somthing? If its outgrown it you should be more concerned about rehoming it than making profit; the only size where a plec starts torealy get more worth in its value is usually when they reach 12inchs, but you would need at least a 50gal to raise your avaerage 12inch plec in and there isn't a huge market for large plecs, its more like a one off customer you wants big fish to fill out his new 150gal tank quickly type of thing.