How hard to install one yourself? I haven't really looked into it, but are they over charging for what seems to be a $50-75 item?
Very easy if you're comfortable working in the electrical panel. My best man at our wedding is an electrician and I had him install one for us while I watched and we shot the shit. When he was done I said, "that's it"? Not a lot to it. New ones appear different then ours so you might want to do a little research.
To explain a bit further, ours mounts on the underside of the breaker panel. The panel is surface mounted in our unfinished basement so that works out fine in our case. It has two lights on it. I was told that when one or both lights go out, that it's time to replace it. If I remove the cover on it, there is a nasty burned electrical component odor. There are sacrificial something or another's in there (that's a highly technical term btw) and obviously some of them have bit the dust. They absorb the surge and sacrifice themselves in the process. That's how ours works but it's been in place for 21 years. Newer ones may work entirely different, I don't know.