I think this an easy and personal decision once a gun comes into my possession.
A unique good to high original condition gun is an "investment" if bought right in the short term. It's bought wrong it's a long term investment. I will shoot it, lots at the range, sparingly in the field.
A "shooter", solid, original to mostly original, might have a pad added, or barrels reblacked, might need checkering freshened up. Try to buy them right, hope to break even + or -.
"Junker" near basket case but gun appeals to me for some reason, willing to go upside down, could be just a great fitting gun or unique in some way and I deem it worth salvaging. Have done this with a couple of guns that come to mind. I took an NH 10ga and fixed it up incrementally, fun project, very satisfying. I use this gun a lot in the field, over 1800 shells fired at crows since Fall 2010. Shot a lot of ducks and geese too and some trap targets. Might have $500 to $600 too much into it. Enjoyment of hunting and shooting it well worth it. Same goes with 32" 3 frame 12ga VH. Total basket case that I had DelGrego redo in 2008.
So bottom line the return on investment with a restoration is in enjoyment of ownership and use not investment return,
There is a tuition charged for enjoying Parker's, we all pay it and perhaps later that education returns some dividends. In the meantime I have fun with them.
http://parkerguns.org/forums/showthread.php?t=4750