Cynthiah, Ugartechea makes nice guns, reliable and well balanced. The company is still in business (in Eibar, Spain) and does a fair trade in the States. A new, non - ejector boxlock in 12 gauge will set you back US$ 1,295.00.
Although the gun is a boxlock, a new stock will still need hand fitting. This gun is not like an 870 with interchangeable parts. If the original stock (or remnants thereof) is not available then a complete custom stock would have to be made - "reasonable price" may not enter that equation.
Contact Ugartechea through a Canadian importer of Spanish guns (such as Les Paul) and see if they can sell you a semi - finished stock. Alternately, contact an American supplier of Ugartechea such as Lion Country Supply and see if they have stocks available. You will still need a stock man to do the final fitting, but that is much less labour than creating an entirely new stock. You do not say where you are, but out west David Henry in Bentley, Ab. is good with stocks (be prepared to wait, he is busy). In Ontario, try Nick Makison.
Sharptail