The nearest field is a 30 mile (nearly 50km) drive for me - and with gas prices being so expensive but even moreso in California, it's $15 in gas just for transportation. I feel bad for my old field back in Colorado, as that field was merely a 3 mile drive, and the next nearest one was 40 miles away - and it seems that in January my old field is coming to a close due to city expansion, but I'm pretty sure they're keeping it under wraps for right now. It'll kill the airsoft community, and the shop that's a couple miles down the road.
Once on the field... BB's I don't go through too often. If I play every weekend, a 1,000ct bag of .40g costs $16-$18, making that a pretty easy 01.6¢ / 01.8¢ per round, which would last me about a month or so. I'm not too picky on getting BEST-PROVEN-BRAND BB's - so I don't worry about shipping. A 10 pack of CO2 will last me about 3-4 weeks, $10 for the pack.
The field I play on now used to be $25, but just went up this month to $30. With that, we've also got $5 all day refills for HPA, however there's some rumors going around that it's about to become $5 per-fill. So if I don't need BB's or CO2, the cost of playing for the day is about $60 with gas, field ticket, and air.
Probably what's killing me is being a content creator that's not making any money. Electricity is expensive in California (basically 2-3x more expensive than everywhere else in the country), so charging up all the GoPro batteries, all the electronics, all of my external batteries, that'll start to add up. Throw in editing software, which I use the full Adobe Suite, that's $50 a month. I use paid music in my videos, $10 for that monthly subscription. Then the sheer amount of time that I have to spend at my computer editing the footage and rendering, it'll start to add up - overall being a content creator, I'll say is about $100 a month.
So how much do would I normally spend on airsoft in a month? About $180. How much do I spend now that my car is back in the shop, and I very well may need to buy a new car or a new engine? Nothin.