I'll give a slightly different opinion from above. Most people tend to focus on what it takes to put up a "rockstar" repeater. If you really want that, want to have something with huge coverage and that is well utilized, then yes it is expensive and a royal PITA. Thus why you tend to have clubs or larger groups doing it.
However, you can certainly put together a decent repeater system without spending a fortune or going out of your way. No, it won't have the coverage area unless you just happen to have an "in" with a tall tower or building, and you may not have many users. But if it serves a purpose for you, then why not?
If you just want something to tinker with, then I would suggest picking a lesser-used band to do so as pairs are often quite hard to come by in some areas as was already mentioned. While major metro areas are packed to the gills everywhere, in many areas 70cm still has room. And the parts are often cheaper and much more manageable than for 2m, especially duplexers.
You can spend as much as you want on this, depends what bells and whistles you are interested in. I have a "backyard repeater" I put together for a few different reasons. First was just because I wanted to tinker with it. Also, most of the people I talk with regularly were on simplex but I didn't want to be tied to my desk. The repeater with a remote base on the simplex frequency let me be anywhere in the neighborhood with a 50mW handheld or several miles in any direction in the car, and keep full control of the system. The controller I chose even gives me the ability to remotely change frequencies on the remote base, in addition to being able to turn it on/off.
My repeater surprises some people. It isn't made with converted commercial gear, and I'm sure wouldn't take heavy usage as a regional machine. But it does what I need and is still running after a few years without any trouble. The controller is a nicer one, $400 IIRC for a 3-port Arcom, but the radios are just Alinco mobiles. Two DR-235s for the 220 repeater, and a DR-135 for the 2M remote base. TX turned down to low, and a small fan on the heat sink of the 220 TX, it does great. I did get a bit lucky - a friend happened to have two 220 duplexers and gave me one. Without that, I may not have ever put the system together, or would have gone for 70cm as it's a lot easier to find gear for that band. 220 is nice, though, as it's essentially dead in my area so plenty of room to tinker.
The coordinating groups vary from place to place, around here it's easy. I just went to their website and filled out the 1-page form. Sent it in, waited a bit, got notice of approval.