$115.00 per sf est of rehab costs?

[Katy Fry]

Hi,  looking at an article I saw on BP, if a house was built in 1902 I would est $1 per sf for every year old for rehab.  That would make this estimate, for a house just under 900 sf, about $90,000 to rehab?  Is this right?  The house is a little one bedroom, has water damage on the walls, lath and plaster construction.   It does need a lot of work but is a solid little house, from the walk through.  I realize an inspection would be done but should I even bother putting in an offer?  The asking price is $48000.00 and wouldn't be worth anywhere near $140,000 (purchase price plus rehab) when done.  Even if I got it for free I doubt it would be worth the rehab costs if I go with this formula.

Is this something a 1st timer shouldn't risk?  I would keep it as a rental at $500-600 per mo.

Am I looking at this the right way?  

Thank you for your time and expertise.