Your spending for this seems fairly low. Not for FIRE purposes, of course - but $300 is chump change for a lot of LARPers. If you can keep spending at that level, and keep it at once a month, I think you're doing pretty well. Watch for the budget creep, though.
My SO does the D&D stuff, and did LARPing for a little bit. His friends go every *week*. They were always inviting him, and he almost got sucked into it - even though he says he didn't enjoy it! The nearest Dystopia games are in New Jersey and somewhere in southern Virginia (both hours away!)
Our sanity was saved by some spendy friends. This couple takes home about 200K and constantly claims to be out of cash. One day SO asked them what their biggest budget-buster was. They didn't talk about the new cars, they didn't talk about the rent or the baby. They said their biggest expense was LARPing. Especially buying clothes for the games: "We have a lot of power in this game, and so we need to dress the part." That was a wake-up call for SO. Instead, he makes a pretty penny babysitting when his friends go LARPing.
Also, I sort of agree with the suggestion to get more involved. You'll have to decide for yourself if it's worth any of your time. Yes, LARPing is absolutely ridiculous, but if it makes him happy....... I know people who don't play, but learned some peripheral things that made their SO feel a little more supported. One lady took a liking to making chain mail garments (armor for the men, tank tops for the ladies) and made a lot of money from her husband's LARP group. And they were excited to give her their money! If you can easily contribute to a small part of his game, why not? He'll have extra food or clothes for power/money/bragging rights (can't tell you how many times I've heard "your *girlfriend* [helps organize/sends cookies/doesn't hate us]??"). You'll learn something, and he can feel like you support him, even if he knows you don't care for the fantasy.
Remember, too, that your SO's LARPing habit is exposing you to a whole new network of people. See if you can host the local LARPers for a [board]game night sometime (out of character!). I have yet to meet a LARPing handyman, but I've met doctors, lawyers, a General, and IT folks. They are generally nice people, and it's not a bad set of skills and resources to have on call. So far I've mostly gotten free rides to/from the airport, and in return I meet their new girlfriends and attempt to explain that dressing up and running around in the woods is nothing to worry about. (Nope, no sugarcoating here!)