I have been growing out fruit trees from seeds and making crosses for about 7 years now and I still think it is the most wonderful thing to do. Making crosses and thinking about what the fruits could look and taste like is such a wonderful and imaginative activity that has never lost it’s initial attraction to me: the fact that you can create something entirely new, alive and uniquely flavored is so enticing!
All the things you study and learn along the way - like genetics and botany - are also just so incredibly interesting, that I can only recommend it to everyone. - As an activity or past-time that is thoroughly enjoyable and interesting. This kind of breeding gives plenty of good, nice enough and interesting results, even in a relatively small garden with small numbers to keep it interesting for a long time. Besides that it is very easy to do if you are willing to do it over a longer period of time and have the curiosity and the patience for it.
Other than that, I would say one should not have any major expectations of it in terms of value or return on investment. Just by doing it for a couple of years, you start to realize a couple of things that all other breeders have already said a hundred times but that somehow are hard to imagine when you start out. And those thing have everything to do with the value, or market value, or investment value, and not at all with all the interesting techniques, novelties and your creativity - if you would want to keep that in mind.
To my experience:
The market value of your new tree has nothing to do with your choice of parents or your cross or the flavor or the appearance of the fruit. Nor with how many seedlings you grew out over how many years and that you selected the best of the best. The only thing that counts is over how many years and against how many other cultivars you have evaluated your new tree in a solid scientific reliable context and how it compares to these other known varieties in yield, disease resistance, climatic conditions and under pest pressure. It is the gathering of this information what will determine for 99% the market value of your new tree, no matter if it is for the home garden or the production market.
This means you, or anyone else will need to set up a program where it compares your best tree or plant over several years against all the market standards. That is the investment that anyone who will bring the tree to market wants to recuperate.
Otherwise, as often stated in this forum by the professional growers, there are plenty of people who swear that the tree in their yard never gets peach leaf curl, or is the sweetest, biggest, most wonderful fruit, and that may be true under their conditions.
But the limiting factor and most costly investment is not the number of plants, terrain or generations of breeding - it is in the quality and reliability of the evaluation of your new proposed plant.
So I see this backyard breeding more the way I see fine art or music - something really interesting and enriching to practice for yourself, but there are many, many easier ways to make a living. As an investment or a plan to make a little money on the side it is has almost no potential at all, in my view, unless you are in the nursery business yourself already, or you want to make this your full time job.
That said, it is very possible, even very likely, that you will find a very good fruit after a while. That fruit can be distributed and gain a certain reputation locally and even become a standard or legendary variety. That is probable and even likely in my opinion. So the door is open to contributing a ne and novel fruit variety for posterity - no problem. It is just not easy to do that and make money from it at the same time.