Ex: I'm embarking on a rehab that includes new windows, sheetrocking, painting, refinishing hardwoods, etc.

Paint maybe...that must be one ugly duck...got any photos?

You need to find out exactly how far they are behind and if it's just on the loan or taxes...make them paint a whole picture for you.

Can you do some low-cost fix-ups like paint, flooring and landscaping?  

If so, is it you do not want to or do you believe you cannot get a (what I call a) true fixer upper (heavy rehab not jsut paint and carpet) at a deep discount because you are using a VA loan?

I would stud pocket in another slider option at the same height to match the existing units...If the windows are in good working order I would simply paint and possibly add maintenance free PVC trim.

On the other hand, I'm living in a gentrifying area where it has it's own unique challenges, not in Manayunk with a pool :) I know this doesn't answer your question about the numbers but hopefully it'll get you thinking bigger picture so you're not painted into a corner when you're ready to move out.

Low-income neighborhood and lower price point has attracted FHA applicants ... and all the FHA hassle that goes with it. 2 buyers fell out due to finances, 1 buyer backed out after the 25 page FHA inspection painted a picture of a house that deserves demolition.

I recommend everyone read this, there's data against hot tubs, sump pumps draining into septic systems and yes even water from latex and other paint brushes going into the septic.

I have done lots of painting, trim work, cabinet installing, etc so far.