Interesting idea that will never ever happen. Your best bet for a dollars to donuts exchange is to do the buy & sell items for XP that will allow you to get to free donut level up bonuses quicker. The weather station and bloodmobile are apparently good items to do this with if you have a lot of extra cash lying around.
And your idea of $5,000 per donut and $750,000 for a 150 donut premium item is way too low. It takes me about 5 days to make that much cash - I'd never buy donuts again if I could accumulate 150 donuts every 5 days by selling my in game cash!