Construction Costs Per Square Foot

The costs really depend on the type of house it is.
You could be as low as $8 a sq. ft.to $25 a ft.
If you are doing the work yourself and it is just a quick paint and carpet, in a low end house, you will be on the low end of the costs.
If you are rehabbing a high end house, you have to consider that you'll be putting in granite counter tops, nicer tiles and hardwoods, as well as a better carpet and much more expensive appliances...
You can usually be safe at $12-15 per foot in most normal houses.
I always put my numbers on the high side. If it ends up costing less, you or your buyer, make more money.
If you can make the numbers work on the higher end of the scale, you know you can make the deal work.
Something else to remember. You almost always have unexpected costs when you rehab a house. Regardless if you've had a thorough inspection done or not. I don't think I've ever rehabbed a house that didn't have some sort of surprise. So, always add in an additional $2000-2500 for these happy little costs.

Good luck,
Jim