Many moons ago I was in the Army, having passed the tests but failed the medical to be an RAF pilot, always wanted to fly but never thought I would be able to because of my eyesight problem.
I took up rallying as a navigator. Because the skills required were beneficial skills to Army service we were able to use Army Landrovers for the rallies. This essentially meant the cost of our weekend hobby was just our time and the entry fees. We had to do the modifications (Fit Rollbars and Lights etc, and do any repairs in our own time as well).
Anyway another squaddie in our unit used to rally in an NSU prinz, the rally car of the era (giving my age away here). When he wrapped it round a tree and spent the next few weeks burning the midnight oil rebuilding it I asked why he was spending all of his money on a rally car when he could use a landrover like we were.
"Because its my hobby. That's what we earn it for, to spend on hobbies."
"Another guy used to spend all his money and use all of his leave travelling to Formula One Races".
"So there's your answer - you don't have justify it, its a hobby, that's why hobbies exist - to soak up spare disposable income".
So when I did start learning to fly (It transpired that my eyesight problem is only a class one problem.) I didn't have to justify the expenditure because its my Hobby.