I look at it primarily from a financial perspective, so for me it depends not only on the machine but how much you value your time.
I do all my own high end restoration work (cabinet decals, new playfield, chrome trim and chrome coin door et al) and I would never consider doing the high end treatment with that amount of labour and monetary outlay involved for any machine that isn't certified by the community as an A list machine unless I had a client/buyer already arranged who was willing to pay the quoted priced on a B list or below machine.
With machines like AFM, TOTAN, CV, MM and the like you can be assured there will always be a market for an "over the top" ground up restoration so you aren't in essence simply pouring money into a bottomless pinball pit.
I doubt I would go to the same labour/ monetary outlay for a Fish Tales as I would for a Medieval Madness.