About 250-300 USD per year. It's changing a bit as hosting costs are going up & down as things like RAM & CPUs are constrained - but the last 6 months or so the hosting is about 15 EUR per month, which is very cheap. I've moved us a few times to keep these costs under control. Software licensing for Invision is another $40 USD per 6 months. I want to get us off Invision because they're just rent seeking at this point and it's not worth it.
Our donations aren't regular but we have enough in the coffers to keep us going for another year or so. If we get off of Invision, it will be closer to 150 per year - the variable cost is hosting and with AI driving up the cost of storage, RAM, CPU etc I would expect this will be volatile for a while.