I think it'd be relatively low compared to the cost of running a game (most games anyway)
Using public nodes for a dapp feels like one of those "let's do that since we can afford it right now and move to having our own once the dapp is generating enough income"
If there were no "free" nodes, I think that'd kill quite a lot of development. I sure know I've made silly projects that I wouldn't even consider if I had to run a Full Node just to launch them.
Say a full node is $60/mo, it's probably reasonable to expect people to pay for it when they're making a couple hundred or more and/or are hammering nodes heavily (lots of transfers/reads)