Hard to say without examining the parts. Certainly you will probably need a new wheel, a new hub, a new rotor and brake pads, maybe a new brake caliper. Do you have insurance? It may cover this.
Usually a wheel falls off because:
1. Someone forgot to tighten it
2. Studs break off the hub
3. wheel bearing seized and disintegrates
Figure your car is worth maybe $2,000--$2,500 if it's in decent shape, and less if it has wear and tear on it, so you have to balance that number with the repair estimate and figure out if your car is worth fixing. Only you know what else is wrong with it. It's hard to find a decent used car these days for less than $4,000--$5,000, so if yours is clean and otherwise in good shape, it might be worth fixing.
This is all straighforward nut and bolt work. Any good shop should be able to give you an estimate. Of course, I'd have it checked out for suspension damage.