I've never done this, but I suspect it will be like pretty much every other DIY project. It looks like it could be cheaper at first, but in the end turns out to be more expensive than projected for any of the reasons DIY projects end up like that.
I don't know. I doubt I can get an 16"-18" mirror for under $300.
Though with how much labor it is, you do have to value your time at $0 pretty much.
Which is fine if you want to make it yourself as part of the hobby. But if your only purpose is to save money...
Free time is effectively free. The question is "don't you have more interesting things to do".
And how many tools are needed that are not currently owned?
That's included. For example, I'm building a kiln to fuse the glass. That's where the biggest cost is.