Your question is a bit vague, @Mary Eubanks. There are many rates of return, such as ROE, ROI, ROA, etc. I assume you mean Return on Investment, like this:
ROI = Net Profit or Loss/Amount Invested
I don't believe this is an appropriate way to evaluate a flip.
Getting money after you have a number of flips under your belt can be fairly easy and all the higher volume and/or higher dollar flippers we know eventually run out of cash. It's pretty common to obtain 100% financing for both the purchase and rehab funds after you've been doing this for a while.
Borrowing, of course, is a two-edged sword because your interest payments, especially for a hard money loan, can eat 1/4 to 1/3 of your profit. Nonetheless, most of the experienced flippers we know will borrow as much as possible to do as many deals as they can and keep the pipeline full.
Obviously, the more you borrow the less your Amount Invested is in the formula above and the higher will be your ROI. If you borrow 100% of the purchase and rehab costs your ROI will be infinite and it doesn't matter if your profit is $100 or $100k. Nor does the cost of the property enter into the calculation. That is, would you buy a million-dollar property to make $100 even if that represented an infinite return on investment?
Similarly, merely stating you expect to earn a fixed minimum of $50k on any flip doesn't make sense either since that could be against a $350k property or a million-dollar deal.
Instead, we use Return on ARV to determine if a flip is a good deal or not. In our case, we define 10 to 12% of the ARV in profit as a good deal. Here:
Return on ARV = Net Profit or Loss/Expect Sale Price of the Completed Property (ARV)
For example, if a property with a $500k ARV looks like it will result in a $50k to $60k profit, we consider that fair. A $1M ARV would have to produce a minimum $100k to $120k. We won't make a loan if it does not look like the rehabber can earn a profit of at least 10 to 12% of the ARV, converted to dollars, of course.
This approach takes both your profit, in dollars, into account as well as the value of the property. It also takes some risk into account because it predicts you make more on the higher-value properties, where price swings and rehab budgets can be greater.
We also use the rule-of-thumb formula above, for our evaluations, and it works. Except, for properties with an ARV greater than about $350k, I can show that the rehabber will earn a profit between about 12% to 15% of the ARV using 75% instead of 70%. Fixed costs take a greater bite for lower dollar properties so you should use 70% for ARVs less than about $350k to earn 12 to 15% of the ARV. It's easy to create a spreadsheet to show this. Or, ask your local friendly hard money lender for the one they use.
Good luck to you, Mary.