Just split the ticket at suitable places. Perth is useful.
Berwick brings the costs down but it depends on the times you are travelling as many trains (including the HC, usually) do not call there.
Then there's the obvious places like Edinburgh, Newcastle, York.
By splitting at all the above, I got the fare down to £46.40 (note: 2xSDS YRK-SHF) but that's only negligbly cheaper than the through fare of £50.75 so probably not worth the hassle!! (note: as usual these are last year's fares from the leaked Avantix Traveller)
I recommend giving Avantix traveller a try (See other topics). When checking fares put "PRIV" in the discount box. The only way to find the cheapest possible way is to spend ages typing in different places to split the tickets.
I recommend going there on a Saturday night and back on a Sunday as the HC makes more calls on weekends and departs later on the Sunday. If you wait until July/August there will be a group of us who can join you.
I also recommend getting a Scottish rover if possible (£35 for 8 in 15 days is an absolute bargain! and cheaper than a Berwick to Inverness ticket that is only valid once!) and spend some time there.
Sods law, if you only go once you'll get a double MTU refurb
It's a great line to get a Valenta on. You have the thrash of the long 1 in 70 and 1 in 60 climbs, reaching the highest point on the UK rail network at
Drumochter (twice the height of the
UK's tallest building!) plus and the stunning scenery.