I've just replaced my router with a good quality new one (a 'customer return' item in Maplin's closing down sale) for £13.50. The one I'd been using up until then had lasted for ages after I bought it for a fiver in an auction (and that price included a high-resolution film scanner as well).
So, given those prices, I doubt very much that old BT Home Hubs are worth much. Indeed, many potential purchasers might be put off buying a product specifically designed for use with one particular broadband service (fearing that it might not work with a different service), rendering it of even less value than a generic secondhand router would be.
Some charity shops offer old routers for sale but they seem to simply sit on the shelves for ages, with no great rush to purchase them. (That's probably because many ISPs provide brand new routers for free anyway. My own ISP charges £84 but I'm probably in a minority in having to seek out a cheaper alternative).