Buy used equipment. There are always people who's misfortune (police or landlord or whatever) enables you to pick up great gear for cheap. We just finished an extra little closet under a bunk bed, that measures 4 feet by 7 feet. Its completely sealed, with an 8" exhaust fan (pulling air from 2 6" hoods) and 6" intake fan, ducting, Y pipes, 2 1000 watt air-cooled HPS lights, a carbon filter, 2 circulating fans, 2 4'x3.5' planter boxes, 2 bags of Fox Farm Ocean Forest, a nice pile of nutrients (micro, grow, bloom, bud juice, growth excel, and vita boost), a watering wand, plastic sheeting, mylar, a 5 gallon bucket for mixing nutrients, CO2 tank and CO2 regulator, and 60 clones. All for under $1000. Obviously I didnt pay anywhere near retail for any of this, but it was all from random people, none of it from friends. The clones were quite a steal, 60 of them for under $200.

Anything is possible. This box cost more then a regular room would too because you wouldn't need $150 worth of wood to build it.