Well it all depends where you are playing. Here on Equus Sims, it is still a little bit up in the air. We are working on something a bit more structured/fun for those of us who do enjoy having set goals and such, so when that is prepared and finalized, there may be ways to earn value for our horses and sims in a way but this is still very much up in the air at the moment. In other words, if you are only playing on this forum, your horses have the value you choose to give them. In other words, if you want your stallion to be an olympic champion and price his stud fee or his offspring accordingly, you can. Just like if you want to start your stallion as a kid's pet have him be worth close to nothing until you show him around in our shows and workshops and up his value from there, you also can.
Now if you play on Equus Community, they have entire complex systems. Basically you start with a foundation horse and enter shows on their forum, earning your horse points. The more points a horse has, the more value it has. I'm not exactly sure if they still function that way, as I haven't played there in over a year but, I believe they still offer pedigree bonuses in shows (if someone better informed on the subject needs to contradict, please do). So basically the more points the stallion has, the more bonuses it can give his offspring and thus his offspring and his stud fee will be valuable. The further you go down the generations, the more valuable they tend to become. Plus in general people tend to love buying a horse who has actual lines created who have competed and made a name for themselves.
Either way, I'll say it's always a good idea to try and enter some shows/workshops or at least post loads of pictures in a scrapbook to get people to know your horses and style before proposing stud fees/selling foal crop. People will generally (and again some will, just not the majority) not buy/breed from members they know nothing about. Having visuals of great looking horses you create and actively participate on the forum will always guarantee more traffic and demand.