I bought dog kennel pans. They aren't that cheap, but very effective. About $50 each for the large ones are Petsmart. Two of them fit perfectly side by side in a 5x5 tent, with about 10 inches or so of room to step into the tent. If you are going to have tables going across the pans, just run a piece of duct tape down the seam to connect the two sides so runoff doesn't miss the pans. They have other sizes that would probably work for smaller tents.
I also bought a bunch of rubber boot mats for $3 on black Friday at Menards, and they are great for containing run off. I've seen pretty cheap plastic ones here and there too for about $4. I use them to set pots on if I'm pulling them out of the tent to rearrange or anything like that.
You could get a garage floor mat. Costco has a 8x20 diamond tread mat for $200. Not the cheapest, but pond liner isn't all that cheap either. But I think that would be a one time, no hassel solution. You wouldn't even have to worry about it ripping.