That kind of depends. Going from .243 to .308 is no big deal. The bolts should be the same. If you hunt around on sites like snipershide.com you could come across a good deal on a pull off barrel. I've sold them for as little as $75. All you really need to do is have a gunsmith screw the barrel onto the action and check the headspace.
If you want new, there are the Bergara barrels which use a Savage like barrel nut for a do it yourself barrel swap. There is a video on their website.
When I rebarrel one of my 700s, it costs me around $500. $300 for a Krieger barrel and $200 for my smith to cut it, thread it and chamber it for me.
If you go the route that I usually take, I'd suggest getting the best barrel that you can, because the labor fees are the same, no matter the barrel.
I'd also suggest checking out Brownell's. I've never looked, but if there is a prethreaded barrel for a 700 out there, they should have one.