I currently own 7 machines. Of these machines, only one was working (Whirlwind) when I bought it. Most of the machines that I have bought were incomplete, mostly missing boards. Bringing a machine back from the dead is what makes me enjoy this hobby the most. I also don't really have the money required to own most of the A title games.
I bought my project BSD missing all the boards in the back box and also the coin and flipper boards. I probably paid more for it than it was worth at the time, but it is hard to find good games here where I live. Over the last 6 months, I have picked up the boards I was missing from Ebay and Mr Pinball. All of the boards that I got were non-working (more challenging/cheaper) and cost me around $250. After buying the parts needed to repair the boards and other missing parts, I may have around $800 in the machine. I think that the machine is worth the money I spent.
I also have a Party Zone that I bought that was missing all the boards, other than the CPU board. I have probably about $700 into it, but there again I bought non-working boards. I think that machine is also worth the money I spent on it.
They both may not be collector quality machines or A title games, but the thing that matters to me is that I enjoyed all the time working on them. If you would buy working or new boards, I really cannot see how it would be worth the money spent. Luckily out of all the non working boards I have purchased for my machines (probably 50+), I have only run into 2 boards that I could not fix (too much acid damage).