that's a pretty broad question man. a lot of variables. soil or hydro? what size plants? you could do 50 SOG plants in soil pretty cheap. 50 big ass trees in an RDWC or Under Current system would be pretty damn expensive. depends to on how much you need to spend to get the room ready...i.e. a/c, fans, construction, etc...
if you go soil and grow some decent sized plants...maybe $10k for everything. room construction, a/c, dehumidifier, lighting, electrical work, etc...that's just a very broad guess though. if you could give more details it would help to narrow it down to a more accurate number.