It should be pretty cheap. All you really need is a big sink and a power outlet.
I know you already have an enlarger, but if you don't have the other accessories (trays, timer, easel, safelight, chemistry containers, etc) those usually are given away with enlarger sales, so you can probably look around for a used enlarger in your local classifieds and once one comes up pick it up plus all the accessories for peanuts.
I got an omega D6XL + dichroic head, 16x20 4-blade easel, 11x14 borderless easel, JOBO CPP-2 w/ lift (described as broken but works perfectly! lucky me), 3 JOBO print drums and one film drum, trays from 5x7 to 16x20, 4 extra enlarger bulbs, safelight, timer, foot pedal, 3 lenses (50/80/135), contact printing frame, and probably $500 worth of B&W paper (8x10, 11x14, 16x20) and some possibly usable cibachrome & RA-4 paper (up to 20x24) for $550. I had to buy an extra lens ($100 for a 150mm componon-S for 4x5) and an easel (11x14 sauders 4-blade). Ignoring the enlarger all the other stuff is probably worth $1500 even in today's used prices (the JOBO alone is probably worth $700-$800). There are great bargains to be found in the local papers on darkroom stuff - everybody is dumping it.