That is a big range! I think it depends on how much of a prospect/project you are looking for. I think going from 4-5 to the 5 or 6-7 range can make a big difference because it is the difference between something that still is growing and learning how to use its body to somthing that is about to move into its prime years. You therefore know more of what you are getting.
FWIW, I’ll show you what I mean with my example:
I bought my 16’3 green-started (W/T/C and some teeny tiny x-rails - just to see how he hadnled them, basically) 3 yo TBxDutchWB (very WB typinsh) for under 5k. At the time, he had potential, and was broke to ride with decent ground manners, but not much else. My trainer and I could tell at 3 that he had the potential I wanted, but that I would have to install all the controls and buttons since he didn’t really know how to work his legs all in the same direction, much less what contact was. I took the chance becaause I could never have afforded a more finished horse with the same raw talent, and when I rode him I could tell he had a good brain.
Two years later at 5 and 17h and growing, he is still a prospect, but he now has steering, can work consistently in a training-level frame (with short bursts of first level), has a big floaty 10 trot, a ridable and ratable canter and he could jump the moon if I asked him to and not even think about it. We’re keeping it low for right now since he is still growning, but currently we are successfully showing 2’6 and schooling up to 2’9 on courses. He’s jumped up to 3’6 as a one off here or there, but that is once in a blue moon as I am building the basics slowly and protecting his legs. However, since he can trot a 3’2 verticle without any effort and I’ve seen him jump about 8 feet across a water puddle from a stand still to keep from getting his feet wet, I’m know he’ll go higher. Basically, the controls are all there now, it is just a matter of refining, improving,adding consistency and overcoming the occassional stupid baby moments. On the ground he can be pushy, but overall is a well-mannerred beast, especially considering that at best, he is still an adolescent.
I think if I wanted to resell him at this point (which I don’t!) I could easily get in the low 5 figures, more if I could ever get a more regular show schedule going instead of the helter skelter every few months thing I have going now. If he continues like he has been doing, I think that once he is done growing at about 6 and I can really hit the throttle with his jumping, his value will increase dramatically.