there are so many factors, but generally, if you are looking at the tractor only, your cost per hp will go down as you go up in size. the questions arise about all the other factors:
do you need a 50hp machine to manicure a 1 acre lot?
will you need to spend an extra $30k on a larger vehicle to tow a $20k tractor.
there's a ton more things to consider too. what i do before any purchase is to evaluate what i think is the best size machine for the job, then compare it to ones slightly larger and smaller to see if there is a benefit to any increase or decrease in size and price. don't forget: as your tractor size increases, the size and price of all the implements will too, so it's a few hundred dollars extra for each one of them too. maybe it's worth it to you, but then again, maybe not.
i try never to go into a purchase looking at the price first. i look at the product i need first, then the price. granted, the difference may be staggering, but if you were set on a $15k price, and found out too late that that a new tractor for that price did not do what you needed, then you've got a whole lot of money spent on something that doesn't work. maybe it would have been better to find a $20k tractor that was a couple of years old that you could get for $15k instead.