If you buy one month in advance and at non-peak times (e.g. 12:00 on a Wednesday or similar), it will be around £30-40 probably.
It’s £150ish open return from Dundee to London via Edinburgh IIRC for standard class off peak.
Pretty much same price as Dundee to Edinburgh train > plane to stanstead > train into liverpool street.
Also takes about the same time..
I've just checked trainline and for the same dates, a train is 4hours 20, will cost £145 for "super off peak single" - and if I'm not careful booking it I'll be standing for that trip. In my case this time I'll also need to get another train from Liverpool St to east london, which is 2 more trains and an extra 30 minutes each way.
I'd much rather take the train, but the costs are often stacked against trains in favour of flying.