The only other users of the AV8B are the Spanish & Italian navies. Neither of which are flush with cash, in fact the exact opposite. As the aircraft is a joint US/UK design, export control issues prevent sale to anyone the UK or US don't like.
The unpalatable truth is that we got for them what the market was prepared to pay. Do we (the UK) think they were worth more? Subjectively, yes, objectively, no. As others have pointed out, due to the vagaries of Resource Account Budgetting as introducted by The Great Financial Genius, holding on to them actually costs money - money that the MoD doesn't have. You can argue the toss as to whether it is real money until the cows come home, but that will not prevent your budget taking a hit.
Lets not have this thread turn into another "Decision to axe Harrier is bonkers..." thread with no end. It's done, all have to live with the consequences.