Not exactly common, but not rare either. It could be a custom shop rifle or just a production 700, so there are too many variables to give a good answer on price with info you have. I have a stainless 700 KS Mountain rifle .375 H&H with a Mcmillan stock that was a Custom Shop Produced rifle. It is an excellent rifle for hunting in Alaska, I took my Kodiak bear with it, and regularly use it for moose hunting and other foul weather hunts. It was made in 1991. For a lot of years 700s chambered in .375 H&H were a custom shop only proposition, but in the 2000s Remington chambered their run of the mill production 700 and 700 XCR in .375 H&H with stainless and plastic synthetic stock. These would be worth less than a custom shop produced rifle. Do you have any pictures ?
I paid 1300 for my custom shop KS Mountain rifle used on Gbroker and was pretty happy, think I got a deal, but I wouldn't pay that for a standard production 700 with plastic stock.
If you are not familiar with the custom shop 700s, a way to tell them apart from production 700s is the lack of markings on the barrel. Generally a custom shop produced 700 will only have the chambering marked and a few small letters (date code), while a production 700 will have "Remington" and address, possibly warnings, inscribed on barrel. Remington 700s with a few exceptions also have a date code on the barrel (if original barrel), so can tell production date from that code. Generally the synthetic custom shop rifles will have a grey stock, while production will have the typical black plastic Remington synthetic. Of course this only applies if in configuration the rifle left the factory in.
Here is info on date codes:
https://remingtonsociety.org/manufacture-dates/
Here is my Custom Shop KS Mountain rifle .375 H&H
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