That is true, you do not have to buy the uniform unless you are going to be an officer. It can be extremely expensive, and it can be done fairly cheap.
The big expenses are the chapeau ($300 new) and sword belt ($150-60 new). You can buy a brand new uniform for about $130, and Chinese made swords can be had for $35-40.
The uniform can be put together much cheaper than that. I have an inexpensive sword; an older chapeau I bought off ebay; a sword belt and uniform jacket that were given to me by the Commandery (most have items that were donated by the families of deceased sir knights); and a pair of black pants that I bought at Belk.
I'll be commander in a few years and will likely buy all new stuff of my own then pass my hand-me-downs to someone else. If you are interested enough to want to join and be part of the action, I'll just about guarantee your Commandery will work with you to get you outfitted.