I'm not a wholesaler but this is what I think is important. You have to know a general idea of what type of work has to be done to the house, and how much its going to cost. Doesn't have to be perfect, but it has to be close. Then you find out what the house would be worth if it were fixed up with comps in the area. Then you subtract the fixed up cost from what the renovations would be to get a number. You have to factor in closing costs for yourself and the investor. After that's all finished, can you sell it and make a profit where the investor gets a discount on the property? Each property is different, sometimes an assignment fee might be 3000, sometimes it might be 30,000. Part of that answer is how good of a deal you can get the property under contract for. Lets take real numbers.
You get a property under contract for 100,000. Fixed up, it will be worth 200,000. You go and look at the property before you make your offer and get it under contract, and you determine it needs 40,000 dollars worth of work. Closing costs for you and the investor are 4000 each. 200,000 - 40,000 - 100,000 - 8000 = 42,000. Then you negotiate from there what hes willing to pay and what your assignment fee will be. You should have a good idea by then.