After a verb,
不起 says you can't do something because you lack the money, strength or standing:
买不起 'can't afford to buy',
惹不起 'can't afford to cross',
经不起 'can't stand up to'. The positive form swaps in
得:
买得起 'can afford'. In this position the
不 loses its tone: mǎibuqǐ.