A mountain bike is an excellent investment for anyone on a low income. It provides transportation, exercise and excitement. Transportation is obvious - you can use it on your commute to work; in a city with even moderate traffic, it can be faster than cars, as you can ride in the space between car + curb, or on the sidewalk, when cars are stopped at lights. Exercise is also obvious: with each ride, you're one step closer to having the calves of Hercules. But the best of all is the excitement: on your commute, dodging traffic is a thrill which can only be bettered by barreling down a path through the woods in a city park at ludicrous speed.
Best of all, once you've made the initial purchase, using your mountain bike is essentially free: the only cost will be food to fuel yourself (and miscellaneous repair costs, of course).