Virtually everything I own is for sale: that is, for the right price, I would sell almost anything. On the other hand, some of the things I own are worth more to me than any other person with any sanity would ever pay me for that thing. The piano I grew up playing and my Dad"s last tractor come to mind.
Something is worth what a willing seller would accept for payment from a willing buyer.
I would sell the property I live on, and have lived on most of my life, if someone would pay me $1 million for it. And I believe I could be off the property in less than one month if that was part of the deal. But that is very unlikely to happen, since the property is probably not worth even half that much. And there are other properties in the area that would sell for much less. Do I want to sell? Not really, but I would for enough money.
As far as tractors go, there might be some basis to value a particular machine based on other sales of similar machines in the general area. My experience is that often when people have lots of time and money in fixing up a machine, they usually would not sell that machine for the "going rate". So they might very well just keep it until someone comes along that thinks the machine is worth more than the guy who owns it.
Your M and GMC look very nice from what I can see from the single photos. Neither is super rare, although I have never seen an M in this area with a single front wheel. I am not familiar with the machine in the 3rd photo, but it looks like a project that is progressing nicely. I have never been very fond of the Wisconsin air cooled engines though.
What are they worth? Really hard to say...probably less than you have invested in them, but you never know. It also depends on how bad you want or need to sell them. I usually quote a high price when someone wants to buy something from me, but have ended up accepting offers that I thought were OK for me. I have also sold things for token amounts or have given things away when I thought the person really needed the item and would probably take good care of it. But that"s just my way.
If I had the pictured machines, I bet that I would just keep and enjoy them! Good luck!