How can I get a good estimate fo ROI for a franchise opportunity?

So I’m looking into a lesser known but established franchise. It’s my first time doing anything like this and I would like to get a good estimate for what my ROI would be.

First I tried calling a bunch of franchise owners They are all very helpful but no one wants to get specific and tell me what their profit margins or net profit is.

So being that I am unable to get historical data from franchises or corporate how can get good data that would let me estimate the ROI?