So looking at this a bit more closely, all of the buildings on offer are priced at 60 donuts or more. Adding the 60 donuts to the 576 gives at least 636 donuts for this offer.
300/20 = 15 donuts/$
636/35 = 18.17 donuts/$
900/50 = 18 donuts/$
So basically it's a deal that offers you a similar donut payout to the $50 offer but you don't have to spend as much. Not a great offer, not a terrible offer...let's call it a "meh" offer as Lisa would do.