You don't have to pay 100$ to run a marathon. If you have a pair of running shoes and a normal running belt (for water and a gel) that will do. You just have to put them on and there you go. I've done a couple of "Sunday marathons" (aka "long aerobic run") with two friends. They have costed me ~2$ for a gel.
Equipment wise, you don't need to pay 100$ for shoes. There are many <300$ shoes from the last season (last year model) on discount pretty much all year round for 60$-80$. I burn 3 pairs of shoes a year, and even though I could afford expensive shoes, I pretty much always end up finding the ones I want for 70$ somewhere online.
Running belts price varies (from $10 to "as much as you want to pay"), but they are a one time investment. I bought a nice neopren one once, and it will probably outlive my children. If you are running enough to be able to "just go and run a marathon", chances are that you already have a running belt to carry a mobile phone, keys, id, money, gels, water, etc. They are just insanely useful.
I run around 8 hours / week, and my expenses are around 300$ per year. Some years I've paid a bit more, e.g. got a heart rate monitor for 30$, running ear phones for 80$, headlight ~20$, I don't have a running watch yet.., nice running shorts and shirts (couple of 100$s), etc. I've also gone to some races, but I've never paid more than 15-20$ for them. You can do half-marathons and marathons for that money, and some of them are non-profit, so you could deduce that from your taxes if it was worth your time. Also, most races typically give you running tshirts, gloves, and what not as a "finisher" gift.
Honestly, factoring the time I've spent running during my lifetime, and the money I've spent over the last 10 years, it's by far the cheapest hobby I have.
EDIT: That might be a lie. My absolute cheapest hobby is actually swimming. Swim shorts for 20-30$, swim googles for 5-10$, and an annual membership on a masters team (120$/year for 6h of swimming / week, where I only actually end up using 2-4 h). Swim short and googles need replacing only every couple of years, so swimming costs me around 150$/year for about half or a quarter of the hours I spend running / week.
Swimming and running, compared with my other hobbies, are negligibly cheap. I go snowboarding for 2 weeks a year, and that cost me ~2000$ + maintaining my equipment (I basically end up spending ~300-600$ on snowboarding equipment / year). I go cycling with my gf every now and then but not enough to make a 2000$ road bike + equipment cheap. And well I have a motorbike because I want to and that also costs multiple 1000$s dollars per year, I don't even want to know how much.
So yeah, swimming, running, calisthenics, etc. are damn cheap sports. You don't need to spend 1000$s/year on these. If you are tight on money you can probably manage 20h of fun per week by spending ~300$ per year without issues. Obviously, if you want 1-on-1 crossfit training wearing fashionable clothes wearing a go pro and drinking avocado toast kale smoothies you are going to end up paying a lot, but that's not necessary at all.