Happened to me and there were two issues:
1. I had about a 6" long crack in my tank, which I had a friend weld up for me.
2. After I got it all back together, I tested it and it leaked again, which turned out to be a bad radiator cap.
I wouldn't say it's common to have a cracked tank. Mine cracked due to rust inside the tank. It is common for the filler neck joint to come loose. There are two ways to fix it. One is to tear it down and braze it back and the other one is to buy a pre-made fix, which people have had good luck with. And the cap is a $5 cap from autozone (or your favorite auto parts store).
What I would do first is pull the cap and see if there's residue on top of the tank around the filler neck. You'll have to remove the rubber cover that sits there. If there is residue, replace the cap with a new one, run it, and see if it leaks. If not, good. If it does, and there's new residue there, it's likely the filler neck. If no residue in the first place, it's another component. Before suspecting a cracked tank, inspect all of the hose connections from the loops and the pumps. I also had a rubber hose connection on the back from one of the loops that started seeping pretty good. Easily replaced with new heater hose and sealant. See my thread here for the entire saga starting with post 54:
Good luck! With a little elbow grease, you can repair it yourself. Just try not to do it in the dead of winter. I was VERY lucky to have indoor storage for the majority of it.