Taking one marathon I've done, shoes (which get used a lot) were £110 (because I have wide feet and have to buy Hoka/Altra); travel was £40 (train to the coast) + £4 (Brompton hire); accommodation was £90 (B&B); entry was £40; kit was about £40; £324 total. If I had a car, I could have swapped £134 of that for about £30 in petrol. The shoes and kit will last for at least a year and should be amortised.
You could probably do a marathon for £130 all told. Which is not nothing but it's a pretty low barrier to entry when you consider the cost of other sports.
> so on that a dedicated runner spends their money on
Anecdata: I know a lot of dedicated runners (50+ marathons/ultras a year, etc.) and almost none of them spend money on specialist accessories, bags, shoes, kit, etc.