The symmetry of of the forearm can be handled by using a scale, and such things as calipers. However you also use the good old eye ball which with a bit of training can be suprisingly accurate. The butt of the rifle is another story, here it is mostly up to using your yye unless you are trying to build a rifle to match another in which case countour patterns are used. Building a custom stock let alone a complete rifle takes many hours. The .366 DGW shown took around 200 pluss hours to create with the action modifications and such, scope and all it would sell for around $5200 hawever that is my rifle and yall ain't gona get it!
Double rifles are a different can of worms and the time involved in building, regulating, and all the other aspects send the prices up.

Now that we are finishing the stock it is time to do the final polishing of the metal and start the rust bluing in the case of this rifle. If you want and have a tank big enough to hold the barreled action and you have a way to heat this tank and bring the water in it to a boil you can rust blue at home. (Probably when the wife is out) True rust blue is the most durable of all the bluing processes. With this type of blue you are converting red iron oxide to black iron oxide, since rust can not rust you have a tough, rust resistant finish which will last for many more years then hot blued finishes. The down side to the rust blue is the time involved to do a good job.

One of the big factors in the price of a custom rifle is the stock blank. Nice wood can run from several hundred dollars to thousands. This English walnut blank on the little Martini was a great buy at $595 and that was 15 years ago.
The blank on the rifle I have been writing about is Claro walnut and was $295.