Nah, you just missed the market changing. The market where, for example, other English players such as Andre Gray (£18.5m), Oxlade Chamberlain (£35m), Danny Drinkwater (£34m), Jordan Pickford (£25m), Michael Keane (£35m), Kyle Walker (£50m), John Stones (£50m) aren't worth what they're bought for either. Shelvey didn't become a £20m player, a £20m player became Shelvey. Either way he's certainly not a £12m player. £12m for even a relatively established English player now gets you someone like Sam Clucas, Will Hughes, or if you're lucky Jack Cork.
Clucas and Hughes are better players than Shelvey - Cork I'm not so sure about. This is all irrelevant anyway, if Rafa is happy to get rid of him then we should do so. I doubt he would sanction a transfer if Shelvey was of any use.