Custom Mauser leaves a lot of room to guestimate. Comercial or converted military, adjustable trigger or two stage millitary? I'm assuming D&T'd. Now 22-250s can be hàrd on barrels so unless you càn get them to let you test fire it you have to think that you are buying the action and stock and if it has a bad barrel you can rebarrel it, if the barrel is good it's a bonus. I'd say that a well done custom stocked Mauser action fully done millitary(bolt handle, trigger, safety, D&T'd) or commercial would be worth $450 just to build on, if the barrel is good it would be a very good deal.
I think builds on a Mauser action are beautifull, I had an originàl comercial Mauser with a Buhmiller barrel in 250-3000, à Herters Presentation grade J-9 in 243 and Mark X Mannlicher stocked 270.