I appreciate it very much and all are great ideas!

Any idea why?

You can do a square footage estimate - but that is only an estimate, and probably not a very accurate one until you've got a bunch of experience under your belt.You can line item all the "stuff" - this takes a very long time when you're getting started, and it's probably not very accurate either, especially when you are pricing at the HD, but a lot materials are going to come from a contractors supply (if using licensed contractors), and labor rates vary greatly.You can try to get a GC to come out and give you an idea of cost - but be aware that GC's don't like this, especially when they show up and find "another cheapskate investor trying to slap lipstick on a pig". 

Even though I don't have the answer for it now, I figured in a couple years I'd have a solution/better idea how to wipe out HELOC.Is it worth taking out this big of interest only HELOC to obtain another rental at this time?

It helps to run these ideas by other more experienced landlords. 

Any ideas about how to calculate the return?

She's saved up a good chunk for retirement + pension and wants to use it against her rehab project, but I don't think it's a good idea and I can't talk her out of it (neither can her sons).Any advice?

I like that idea instead of doing my own search which I was going to do and experiencing the up and downs of it. 

There are many options for you, and I would also suggest that you check out the Biggerpockets Money show as they give you ideas on how to live more frugal and save.

Brrrr is tough since everyone does it, plus construction and contractors are booking right now.The lower budget stuff gets gobbled up by people that have no idea what they're doing or sellers won't take low-ball offers.