The cost depends on what you want to be able to do. I know some people just use spare room in a fridge, a small co2 tank, regulator, and picnic taps. That is going to be relatively cheap.
I also know friends that have True 3 door kegerators with multiple towers and forward sealing faucets.
My personal setup is even more expensive, but I've been kegging and brewing for many years now, and have accumulated a lot of equipment.
Why don't we start with what your budget is, and what related equipment you already have. Then we can give you some more specific suggestions on what you can do for the short term, as well as plan for the long-term.
Walk-In Serving(14): Oatmeal Stout, Hard Cider, Belgian Tripel, Liquid Stupid, Belgian Dark Strong, Red Bull, Rye IPA, Double IPA, Imperial Stout, Irish Red, Scottish 70/-, Belgian Wit, Nut Brown, Traditional Mead
Walk-In Cold Storage(~50): Cysers, Braggot, Melomels, Ciders, Traditional Meads, Persimmon wines, Dry Stout, barleywine, cream ale, sodas, Chocolate stout, milk stout, and others I can't recall.
Fermenters (0): Nothing current.