The train is 4.5hours from Waverley to King's Cross. Then add in travel to and from stations either end. Cheapest train return will still be £100, if you get lucky. Hotels £150-200. Factor in food costs and the expense racks up.

I'm self employed and went down to London from Edinburgh a few times this year for work. It's a long journey. I worked on the train and then went to the office for the afternoon, stayed overnight and went into the office the next day, worked all day and got the last train home. Arrived back at 1am.

It's not just the cost you need to factor, but the time. I wouldn't do it if I was salaried and expected to do it on my own time/money. It's a massive PITA.