If you want fine dining on a train in Scotland then Belmond's Royal Scotsman is really your only option, but yes it's prices are eye watering.
There are other Charter trains with fine dining come up on day trips from England- but they have to be booked as round trips.
Very, very rarely there are 'land cruise' trains from London with fine dining. When they run they would usually be around £400 to £800 a day- so not cheap, but rather more affordable.
This is a bit hard to read, but is the full list of UK railtours in 2023- plod through it and you will see who typically does multi day Scotland itineraries, then early next year look at their websites for 2024 tours.- http://www.railtourinfo.co.uk/tours.html
On a normal LNER service train to Inverness or Aberdeen you will get dining, but it certainly isn't fine dining.