Does cheap parking exist around New York City?

We are spending two weeks in NYC in August. We want to stay in SoHo, so of course; there is no parking with our accommodation! We are happy to park out of the city; we don't mind if it is 2-3 hours out by train or bus if need be. We just don't want to pay $25-$35 per day! Any suggestions would be amazing. We were considering finding parking in Boston & catching the bus in if it's easier.