So I collect Dakotas and I recently sold an as-new Dakota on one of the credible websites listed above.
They vary greatly in quality and value.
The top sellers will always be the storied cartridges. 500 Jefferey, 505 Gibbs, and 404 Jefferey will carry a huge premium. Then the 375HH and 458 win/lott will be second. The small bore "gentleman's calibers" will be third with the 7x57, 257 roberts, and 6.5x55 swede. The bottom of the barrel nearly unsaleable will be the Dakota calibers that were proprietary wildcats.
The configuration of the gun, all the upgrades, and the grade of wood effect price. (e.g. XXX english walnut is better than XX turkish) Quarter ribs, color case hardening, and included optics change things. So do hinged grip caps, pop up night front sight, full tanks on the bottom or top, skeletonized butt plates, etc.
Mine was about a $10,000 gun in the Dakota configurator. As new, never fired. Built during the Don Allen era of most demand. In 416 rigby with many upgrades making it very desirable. Had all the paperwork and order sheets. I believe it sold in 45 days for around $5000.
A 416 remington would be easily $1500 less than a rigby all things equal. The 300H&H would be about parity with the 416 remington.
I'd have to see the gun to give an opinion, but it could be anywhere from $1800 scoped for the trash wood or plastic stocked ones as far as value, all the way up to $8000 if it was originally a $20,000 custom gun with every option, highly engraved.
