I think a lot of us have been in this situation.
In fact, over the years I have thought about putting together an AR-10 type rifle but I didn't because I have read too many stories like this one. Part of the problem for me is that I can't just use whatever parts appeal to me. Things arn't as interchangable as they are on a 5.56 AR.
OK, my money pit story:
I read an article once, years ago, about Chinese M-14 clone rifles. According to the article, pretty much everything on the rifle is junk. BUT, the receiver itself is of superb quality. According to the article, better than Springfield Armory and they actually went so far as to say they were better than USGI M-14 receivers. Ironically, not long after I read the article, a buddy called me and said someone he knows is going through a divorce and is selling all his guns. I said, what's he got and among the stuff was a Chinese M14 clone.......for $75. So I bought it.
Year after year, the thing sat around but every time I went to a gun show, I would buy some USGI part for the eventual build. Finally a couple years ago, I decided this thing has sat around here long enough and I sent the rifle and all the parts to a gunsmith.
Well........there were a lot of parts I didn't have. I knew there were some, but they add up fast; especially when you want real USGI M14 parts. Then they tell me the barrel I bought wasn't new: there was nothing wrong with it but it was a used barrel (I thought it was new), so I figured: I am this deep into it so give me a new barrel.
Long story even longer, it cost me less than a boutique M14 clone rifle, but not by much. Plus, I got it back and have never even fired it. Over the period of a decade, I totally lost interest in it.