Generally the rule of thumb used to be 10 percent of the purchase price for something you wanted to hold up and be competitive, back when barn calls were $35. Now I think 10-20 percent is just barely safe for a pleasure horse…
Also depends on age, location, and how much you know the vet. If you know your vet, they can sidebar you before going too far about what they see and their opinion on proceeding. If it’s a new vet because it’s a different area, maybe not because there isn’t a relationship with the vet. Around here a big’un like that won’t be mid-4s and broke, no chance. Additionally a standard does-it-have-two-eyes-and-do-they-work (and some flexions sure) PPE is going to be minimum $300ish. If I lived in a place where I could score a big broke animal for mid-4s, I’d be happy to do more on the PPE because the cost would probably be less and I could afford some pictures and etceteras.
I don’t really buy into the whole thought behind paying more for a PPE because you are paying to avoid long term maintenance costs etc etc. Lets face it, the horse could break its leg in the field the second week you have it, puncture a joint rolling on a stick, get EPM, etc. A PPE is a snapshot of the horse on that day, at that point in time. They are horses and horses rarely if ever get the memo that we want them to not damage themselves and cost us a gazillion dollars. I’m quite stuck with a nice horse who would have passed a PPE (and in fact did with flying colors two years after I bought him as a 2yo when I felt a workup would show future suitability now that he was started under saddle) but is injury-prone and high-maintenance to the point that I can’t sell him because of all the disclosures at this point and still feel good about myself, and most of the time can’t ride him because of some new injury, and costs the most in terms of regular maintenance like injections and four shoes so I can feel good about riding him when I do get to ride him. He’s 13 and gorgeous and it’s very depressing.