The people who enter this contest are doing so with their eyes wide open, accepting the risk/reward payoff of doing so (I'm no expert, but I suspect $30000 for a singe ship model is likely significantly above market value). They know what they're getting into, and they've obviously decided to accept that. Remember that contests like this aren't the primary source of assets for the game - they almost certainly have either staff artists or they've commissioned contract work for the vast majority if what they'll need. This contest is over and above that, and open to people who either weren't good enough or lucky enough to get access to that work. From Star Citizen's perspective, this is throwing money at advertising the game, and at community interaction, not asset acquisition.
As for it being backers money, well, that's a whole separate argument. Personally, I don't like the kickstarter model - it seems very much open to abuse, and frankly I'm uncomfortable with giving away money for either nothing, or for a product of uncertain quality. As such, I personally don't generally contribute to them as a general rule (HLPs drive for the SCP one was an exception, since we were basically paying for coding services). However, when people chose to back Chris Roberts for Star Citizen, they made a decision to demonstrate a level of trust in him and his company. The backers have said "here's my money, make this game", and surely that gives him the right to spend that money as he sees fit, within the confines of making the game that is? If he decides that the advertising this contest gives, or the goodwill it'll generate from the community is worth the cost of putting it on, then that's his call to make, not yours, surely?