I've just checked, and Invisionfree remains free regardless of how big one's forum gets.
You only have to pay if you want to get rid of adverts (and that's about $5 per 30,000 or so clicks - so the more active your forum, the more frequently you have to pay to remove the adverts.)
I used to pay for ad removal for a couple of years ($5 lasted about 3 weeks as I recall), but when it looked like I was going to be paying more often I stopped, and the funny thing is, the spaces for ads returned, but they are now blank, or just advertise Invisionfree itself.