It would depend on the rules where you intend to pull, we go by a set of rules that were wrote for a few clubs in this region that pull across several states, to try and somewhat limit cost, such as must use factory manifolds, no billet heads or grafted on inline injection pumps, other places have different limitations.
It also depends if you want to win or not. You would probably need $15,000 or so to be competitive unless you pull with a group that allows no internal modifications. We have tractors pulling with far less invested. If we had to build a tractor as pictured for somebody I am guessing in the $20,000 range, it just varies so much depending on the rules. Some people have different tastes as to level of fit and finish as you see going to any truck or tractor pull, we do a lot of stuff for the low end people also.