If you are worried about a few dollars for dies , brass or bullets I don't think you are ready for the expense of a custom rifle yet. From your questions I really think you need to do a lot more research as to what is out there before you make a bunch of dollars mistake. If a few dollars for a die or brass concerns you then you do not need to make a custom rifle mistake.
I have tons of rifles in all kinds of wildcat calibers so I don't have to be picky with what I build. But if I saved for one rifle I would know all the answers before I spent my money. First is what I planned to do with it. First is hunting or targets. If I had to do one long range hunter it would be off the 378 wby, big rigby case or similar. 416 rigby brass is cheaper than ultramag if that concerns you. The first time you sit next to me or someone with one of these and you see the difference between 300 fps you are going to want to sell your ultra and get one. Pure and simple. If you are building a target rifle I would go with the 338-300 ultramag. It is cheaper to shoot a bunch at the range all the time and velocity is not as important as how good you can shoot it.
I have at least ten 338's off the 378 or 416 rigby and there is no comparison to the 338-300 ultramag of which I have I think three remaining now. Some I have set up to shoot the 200 grain ballistic tip for hunting out to about 700 yards. Beyond that I have others set up to shoot the 300 matchking at extreme ranges. The 200 ballistic tip at 3500-3700 fps is incredible on elk size critters which is where my guns shoot this bullet. It falls flat beyond 700 yards though and that is 300 matchking country. The 338-300 ultramag will not begin to approach these velocities or do what the big boys will. That means a lot in hunting.
Predictions are difficult, especially when they involve the future