It depends on how big of one you want. You can walk into Home Depot, Sears, Horrible Fright (Harbor Freight) and buy some nice options with drawers, Wheels, butch block top etc for $200-600 and honestly for 95% of jobs they are great. The wheels are handy if you decide to move stuff around etc. if you want to mount a bench Vice, grinder, etc, you need it secured so that changes the plan a little. Then just build one and I suggest locking it into the walls with either small lag bolts or cabinetry screws, there are other options, that's just my preference. I'm going to modify the one built into my garage by the previous owner, it's super deep, which just lets stuff get out of reach, I'm going to bring it down to a 30 inch depth and move it to another wall.
So as with everything in life, it's a choice and you have to decide what makes the most sense for you. It's cheaper to build one, but there are plenty of reasons to buy one too. Good luck!
