What might be a ballpark figure to have a pre-turned barrel chambered, threaded and fitted up to a 700 remington?  I'm starting to save up for a long-range rifle job.

In for information.  I've got a .270 that I'm interested in turning into a long action .308.

If you are going with a prefit barrel, the price should be pretty low.  ~ $165 will get you a barrel vise and action wrench for the rem 700.  All you need beyond that is the headspace gauges.  Some prefit barrels come with the headspace gauges.  Bottom line, I wouldn't pay more that $200 to have  a pre-fit installed.  
The upside of doing it yourself is control.  If you are serious about long range shooting, you are reloading.  So you should fit the barrel yourself.  If you aren't reloading, buy a stock accuracy rifle vs. having a prefit installed on a rem 700 action.

$250-300, which includes threading for the receiver, chambering, install, and headspacing.  Your other option is to buy a prefit Remington barrel with barrel nut, similar to a Savage style.

I did a Pre-fit short chambered barrel for my Mauser build.

Barrel comes threaded and profiled, screw it on, tighten, rent the reamer and headspace gauges for $50, hand ream to the go gauge. Short chambers are usually only 10-20 thou short, very easy to hand ream, the reamer is pretty much self guiding at that point.

Careful though, its a gateway to a rabbit hole, can lead to purchasing a lathe, hundreds of dollars in tooling, and cravings for other machine tools like milling machines and surface grinders!

Many custom gunsmiths won't install a barrel without blueprinting the receiver as part of the process. Money well spent if you ask me.

Every top tier gunsmith has a backlog, sometimes years long, usually at least six months.

Pac-Nor, Douglas, Warner Tool, Master Class Stocks, and dozens of other well-respected companies deserve your consideration. A local yocal, perhaps not.