If you think financials might be a problem, talk with the bride about what expenses you might incur before accepting. Bridesmaids typically pay for their own attire (including any alterations), hair, makeup, and any travel expenses. Though the maid of honor may have a bigger time commitment, there is not necessarily a greater monetary obligation.
Other than illness, family emergency, or an iron-clad work demand, it’s not acceptable to back out once you’ve committed. If you have no choice but to cancel, it’s important to let the bride know as soon as possible.