Honestly... there is really no "magic number"
I approach offers like this:
How much will it cost to fix up the place?
How much do I need to make when I sell the home based on current market data?
How much will is cost me to sell the home (closing costs)?
Add those together... now look at the price of the home... and base your offer on that data.
Example. Let's say a home is for sale for $50,000. It is worth $85,000 it it were fully fixed up.
Lets say it will cost you $6500 in seller's closing costs to sell it
It needs $15,000 in repairs
And you would like to see a $20,000 return on your investment
$85,000 - $15,000 - $6500 - $20,000 = $43,500. So if you can buy that $50,000 home for no more than $43,500... You have a winner :)
Just my 2 cents :)