I don't know how much money is forum exactly spending for maintaining everything running smooth, but I could say the same thing for any other (non-bitcoin) forum that exist today.
How do you think they are managing to stay alive and survive even without having any Bitcoins in their wallets?
Assuming the forum software is free, since the version has been discontinued eons ago, there are hosting plans and CDN (Cloudflare) services although there are custom-priced ones to accommodate the traffic, monthly expenses might reach several hundred dollars. Bitcointalk is about the traffic so there are no SaaS or funds here to guard against threats.
We haven't factored in mods and staffs payrolls yet, but those are a portion from the ad sales IIRC. Current funds should be more than sufficient to sustain the entire operation even after we die.