I'm pretty sure (99.999% sure) the site won't guarantee prizes that are offered by member TDs. To do so would open them up to too much liability; everyone could promise prizes and then just not deliver.
Though, it would be a neat feature if a TD could purchase a prize like that and attach it to a tournament. Not sure how widely it would be used so it might not be worth the coding effort to support something like that.
Looks like you're stuffed there Tom mate, bad luck!! 
Chess.com naturally won't be able to fulfil a promise they didn't make I wouldn't have thought.. but like Martin_Stahl said, that would be quite a cool feature for the TD to be able to attach a prize in advance to the tournament, although I'm not sure the majority of TDs quite have the generosity to be effectively handing out reasonable sums of money to their tournament winners...
OG
Hi
I recently won this tournament: http://www.chess.com/tournament/dokaris-tournament-gold-membership-trophy
The user who set up the tournament was offering 1 year gold membership, but has since been inactive since June last year.
Quick question is this: Do chess.com give out the promised prize or is it a situation that is unavoidable and you'll have to settle for the tournament points.
Also in the future for a similar situation, I think the prize should be given to chess.com before the tournament starts, and chess.com issue it at the end. These kinds of tournaments are of course very rare but just a suggestion.
Regards
Tom