If that gave you access to better features
I'd pay more if it bought you valuable options.
More comfortable seats would be a nice optional upgrade. The Prius seats are not bad, but there are better.
Factory leather, if you like that.
Computer upgrades that improve the indash screen, and allow you to tie your laptop and do interesting things (like the florida accessories guy Dave does, but better)
Or a navigator/computer that allows you to custom program and upgrade easily. I get the impression that this system is pretty much stuck in what it is when you get it, with little room for software improvements.
Upgrades to the suspension/handling, for less sway, better tracking like the European models?
As it stands, Toyota isn't really custom-making the cars you order, just making standards with packages with some attention to dealer requests, and then the dealer grabs one like your order when it comes in. Maybe that's part of the cost-effective mfg. formula that is necessary. Toyota should be praised for keeping the general price point this low!