A lot of pragmatic advice here from those that have done the noble work of restoring a piece of history. We appreciate the seemingly endless effort, which truth be told is driven by the heart-felt love of it.
Another factor that can drive up those frightening and fatal restoration estimates, I think, is the assumption that any worthwhile project must be carried to an overly-high standard. For many private and museum restorations a reasonable goal is limited operation that considers the likely operating venue.
Not all restored locos must, or should, be prepared with the expectation of making long mainline excursions or hauling daily tourist trains. While boilers must be completely sound and legal, shooting for a limited scope of operation would reduce the cost on the mechanical side of things.
Perhaps those considering a restoration should ask: Must the drivers be re-tired to run the loco for photo weekends totalling a few dozen miles a year? Is a complete down-to-the-frame rebuild needed to bring a basically-sound engine back to limited operation? Must the valve gear be replaced and machined back to original specs just to operate an old kettle on a short museum track a few days a year? Perhaps more equipment (locos and rolling stock) can be restored if it's operation after restoration were limited, reasonably.
First hand example - - when we restored little GWEN (Wow - completed 23 years ago!) the drivers could have been turned to like-new profile, the rods, eccentrics and straps could have been remade back to like-new. But where was this little loco going to go, really? Considering that we wanted to demonstrate her at special events (e.g., CSRR Museum Railfairs), and operate on private track on occasion - that level of restoration didn't make sense. If those mechanical rebuild costs were a prerequisite to this restoration - she'd be (I'm afraid) stuffed and mounted rather than enjoyed in operation and preserved in the video library of many fans.
Just MHO.
Bob of AZ