You're rather astute on this point. Patents don't just sprout money. All they do is give you the opportunity to prevent others from making, using, selling, or importing the thing described by the claims. And, you can prevent anyone from doing anything by merely wishing it; you actually have to take legal action and devote the time, money, and resources that legal action requires. Most people not familiar with patents don't realize that.
Your inquiry then is, what is the value that you can extract yourself from such a right to exclude others? Of course, that depends on whether you intend to market your invention yourself (what some call the venture route) or to get others to pay for the right to do that (what some call the license route). In the license route, you don't absorb much risk yourself (risk in terms of spending money that might not generate any income), but you will have a difficult time getting others to take on the risk and pay you for the right to take on the risk. In the venture route, you will take on substantial risk but will likely be able to ensure that your invention has its day in the marketplace.
The analysis you're looking for is really a job for an MBA, not a patent attorney/agent. It's a bit like asking someone who builds houses to help you sell your house. It's an entirely different skill to build IP and to sell (market, evaluate, etc.) IP. Although, knowing how to build something helps you evaluate its strenghth. However, I'll try my best to lay out what the inquiry looks like (as far as I can tell).
Start with one product in one market. Is your invention already there? If so, and you have a patent, you're in luck. Otherwise, what advantage can you give to one or more of the competitors in that market for that product? How much is that worth? You can think per item, or in the aggregate. How much of that can you collect and how much do you have to let the producer/seller keep as an incentive to adopt your technology? Think about whether you can do that for more than one competitor in the market. Does your invention give one competitor an advantage over the others? or does your invention represent a disruptive technology which provides an advantage for the entire market over a competing market? If the latter, repeat the above inquiry for all players in the market for that product.
Now, repeat that for other products in the same market. Then, repeat that for as many markets as your invention pertains to.
That's pretty much it, as I understand it. If you're going the venture route, you should have already done that inquiry. If you're going the license route, your potential licensees will do that inquiry, but it's to your advantage to do it too so that you can convince them that proper analysis suggests they should take a license.
I hope that gives you some idea as to the task before you and I hope it's helpful. I don't know of any specific resources to guide you through that inquiry.
Good luck!