You might post this over on the WeatherbyNation forum. I'm active over there and there's a lot of Weatherby-specific knowledge to be found.
I have several Weatherby rifles. Some are Vanguards and some are Mark Vs. There are lots of differences between models, years of manufacture, and country of origin. The 300 Wby Mag is one of the more popular Weatherby chamberings so therefore it is a bit more common than some of the others.
Without more details, pics, etc. I couldn't really offer much to accurately gauge a value. I'll say this for reference: I gave $700 about 5 years ago for a Mark V chambered in 7mm Wby Mag. It had a factory plastic stock, which I quickly changed out to wood. It was in very good condition with very light use. It included scope bases and rings but no mount. It also included a cheap nylon sling.
Hope that helps.