There are two options to try, before going for a new barrel, or relining the existing barrel.
Shoot it! As is, well cleaned. The rifle may not be capable of decent accuracy with the current bore condition, or it may be. Some rifles still shoot acceptably well with bores that look like plowed fields. Take it out and shoot it a bit with different ammo. It may just surprise you and shoot better than it looks.
Or, it may not, and if not, all you are out is the cost of some ammo and time.
Second option, short of rebarreling...
once it is proven that the barrel is too worn to group with jacketed ammo, consider cast bullets.
Properly made (cast and sized) bullets fit to the specific rifle bore can deliver surprising accuracy from a "shot out" barrel. What you can't get from cast bullets is full jacketed bullet velocity and power, but velocities up to as much as 2200fps can be had from careful cast bullet loads without leading issues. (the devil is in the details, but it can be done).
It is possible that you could turn that worn out Mauser into an accurate rifle with nearly the power of a .30-30 just from using tailored handloads and cast bullets. Not a guarantee, but a possibility, and something to consider before you shell out for a new barrel.
All else being equal (and it almost never is) bigger bullets tend to work better.