In brief, the only return is freedom.
There is no ROI unless you can sell your investment and start a new. Think of it as transportation. You have shoes, they work, or you could get a skate board, then a bike, onto a scooter, a Yugo, a nicer car, then a brand new car.
None of which have any ROI in them, but it's all about what you need vs what you want.
Fact is, I've probably spent close to a half million for my life style, granted, much was a live and learn kind of loss, but there's no doubt about it, it costs a lot to go off the grid if you aren't willing to sacrifice an awful lot of conveniences.
Keep in mind, the costs are nearly half what they were when I started over 3 decades ago, but only because of failed "Green" ventures on the taxpayers back.
In fact, I just picked up a couple of DC Solar's scam. They had built a huge corporation on defrauding investors and the Fed. The trailer I bought will power a home all summer long without sacrificing much at all.
If I were to build one of these solar trailers it would probably cost anywhere from 50 to 100 grand, depending on batteries. But I picked them up for less than $8 grand and they were never even put into use.
Anyway, I didn't really have the luxury of buying used back in the day, with the exception of the defunct ARCO solar panels from a failed Fed experiment from the Carter era.
If I were to give any advice, research, then research some more and experiment with a small system before you take the dive, unless you're already off grid.
And if you are, buy a 7000 watt diesel generator, (I like the Kubota Lowboy 7000 for around $5.700), then, the most batteries you can afford, buy a 2500 watt AIMS inverter with built in charger. This will keep you from killing your battery bank.
Then you can start building your solar array. Too many people think they only need solar so they start there, buy a few car batteries and a shitty off the shelf kragen auto parts 12 volt inverter and run it all into the ground the first year. People tend to forget the little things, like water, a sewage system, Internet. You no longer have access to city services, so this is just one more cost incurred as you venture off grid.
Batteries to start should be 10 or 20, L-16 type batteries (I like Trojan batteries), this will be several thousand dollars, but should last you at least ten years and will run virtually everything you plug into it, just don't run the batteries below 70%, anymore is considered a cycle, and the batteries are only good for 300 cycles.
Running your battery bank dead (10 volts on a 12 volt system) is a full cycle, you could do this everyday, and in a year you'd be forced to replace all the batteries.
As to panels, only buy a single cut crystal cell (Mono Crystal), the rest are essentially crap, but they still work, but by all means, avoid Chinese crap, stay with American, German, Japanese and some Indian/Mexico cells. Oh, and you may want to stay away from Tesla's power wall, though they work, and you can find some refurbished units out there, they're still just AA type electronics batteries, hundreds of them, and when they fail, you have to go through them one by one and test them all.
To date the best battery application is still lead acid, but we're finally approaching the day that all changes, but probably not viable for another 30 years.
Point is, there is nothing cheap about going off the grid, but the more you can sacrifice as in the way of comfort and conveniences, you could just buy raw land with a creek and a couple of shipping containers and call it a day. If I had to do it all over, that's the route I'd have taken, four or more containers, attach them together, cut out some walls and build inside.
Sadly, that's only just scratching the surface, oh, and go with propane everyway possible, including refrigerator, lights to heat in the winter, instantaneous water heater.
