In actuality, the foam tires last alot longer as they get smaller in diameter. They also get a little more firm as they approach the rim. Also, the smaller they are the longer you need to keep tire compound in them in order to keep traction the entire race.
There are advantages to using smaller tires such as less sprung weight, less rotating mass, and faster spin-up for acceleration, but again you need to be budget-minded too.
If you want them to last, start at 2.28, and skim them after every race day. Also, remeber to rotate them every round so one doesn't wear faster or unevenly.
Greg