So I have done some searching and can't find anything. How does one calculate how much buying a pre-trained animal costs? Is it possible to buy an animal already trained? This might make sense for APs where downtime is limited. For instance I might want to buy a mule to carry my stuff. A mule trained for riding (come, heel, stay) would work excellently. Does the 8 gp price include this? What rulebook is this information in? A lot of the forms I have read didn't seem to reference any books.
I think the general assumption is that stock animals are essentially untrained. Broken (as in they probably don't buck you off their back), but not trained to do "tricks".
According to the Handle Animal skill, it takes a week to teach one trick. Your animal will have up to 3 or 6 tricks based on its INT score and how much training it has received.
According to the equipment chapter, the price for a trained hireling is 3sp per day, so it's reasonable to assume that hiring an animal trainer to train your animal for you would cost 3sp per day x 7 days = 21sp per trick.
It's reasonable to assume that if you want a trained animal in a city or other reasonably large market AND the training you want is very common stuff for that kind of animal, then you should be likely to find some pre-trained, but the price is a minimum of the same 21sp per trick, and there might be a "mark-up" on top of that. Maybe up to a convenient 3gp per trick seems reasonable.
Now, for mules taught to come, heel, and stay, this makes total sense - buy a trained mule for the usual price of a mule + 9gp for the training and it's good. But if you want a mule to bury, entertain, and menace, well, you probably will never find a pre-trained mule that does those things so you better train it yourself or hire that trainer (3sp per day) to train your mule.
Side note, I would greatly increase the price for dangerous animals - I doubt you can find anyone who would train a dire bear for just 3 SP per day.
That makes a lot of sense and is reasonable. Thank you!
Any time!
When you look for "reasonable", look no further than your friendly neighborhood tarrasque - our "reasonable" is armor plated!