Interarms Mark X's are commercial mauser actions. Solid, but not a top of the line custom. Douglas barrels are good, but not top of the line either. 358 STA is an 8mm mag necked up to 35, not a well known round. A reloader's rifle for sure. And a lot of punishment on both ends when the trigger is pulled.
The scope is an older Leupold Vari-X III 6.5-20, worth probably $450-500.
Mcmillan stock certainly adds value.
I had not heard of Daniel Cowen, riflesmith, but if he is well-known and respected that can add some value.
The magna-porting deducts value to most hunters, but with a 26" barrel, you could lop it off and recrown.
My guess is $1250-1500 with the scope is what its worth, but it might take a bit of time to find a buyer. Good luck!