I think I have already moaned about the complete illogicality of train fares. Recently I was quoted £19.85 for single back from Birmngham while if I also bought an advance single to Birmngham for the same day for £3, the return leg was only £9.85. 
When DH has a hospital clinic appointment in London (about 2 per month, can be more, rarely less,)I try to pare our costs down but can't do advance fares back as timing is unpredictable.
I drive us to the station, parking is either £10 per day or £8 if I am lucky, we both have Senior Railcards but have to use Virgin trains so that I can reserve a seat as he can't stand, and our off-peak returns, with the discount are £10.50 each. Of course traveling off peak can mean we have to kick our heels in London until the first train after 7 if a late running clnic means we can't get to Euston before 4.
He has to use a taxi to get him to Hampstead, usually between £14-15 in a black cab, but bookng a minicab back brings that down to £10.
So not allowing for coffees or lunch or an early supper if we have to wait until after 7, it works out at over £55 not counting petrol to and from the station (8 miles each way) and we get home about 8.30, having left at 9.
In a way, the train fares don't seem too bad compared to all the rest of it, but there is rarely change out of £75 for each visit, money I would much rather have in my purse to spend on a weekend away once in a while!