Is that the LOWEST cash deal in the local area for a house similar to that one?
Since there is obvious work that needs to be done, when you are searching for comps, you want to look at recent transactions back 90 days, and want to go with the lowest amount. From, there, pick a couple to base your amount off of.
Also, your $50,000 offer is MUCH less than what the owner is seeking. Just because an owner states that they want $70,000 OBO, doesn't always mean they will take $50,000 or something similar. If you stick with this price, explain to the seller, the reasons for such a low offer.
If he declines, shave a little off your commission, and take a lesser one. One of the biggest things I am learning from Drew Downs is that you get rich off skinny deals.
The two sites that I have been using for comps are:
http://www.eppraisal.com/ - I use this site to find out what the home is worth. Also, if you type in an address, and scroll near the bottom, it will give you recently sold transactions near that house. However, you have to do your own due diligence to determine if the houses they show, are similar or not to the house you are about to potentially flip.
http://www.redfin.com - This site, I am also utilizing for comps in the local area. I like this site more for recent transaction, because you can specify 3 months, you can put in the square footage, lot size, bedrooms, bathrooms, etc. When you go to type in an address, be sure to click on "more options" to input better search criteria.
http://www.rentometer.com/ - This site I will be using, to determine what the rents are like for the area.
I was told yesterday from another wholesaler on facebook that end buyers will do their own due diligence, etc, but I personally like to earn what I make, and I figure, if I go above and beyond, the more they will buy from me.
Like I said, all you can do is offer what you think is right. If he doesn't agree, give him a little extra.