The folks in this forum were kind enough to give me advice as to how to do this so wanted to share my real life experience.
My friend was removing her toilet tank to replace the bolts and washers (slight leak) and managed to break off a huge chunk from one side of the tank -- about 8" long by 3" deep. Since it was a 3.5 gallon tank with no replacements available, choice was to glue it or replace the toilet. The break was above the water line (OK, I did lower the water in the tank a little to make that so) and on the side of the tank facing the sliding glass doors on the tub.
Taking the advice here, I used a clear waterproof exoxy, $2.98 worth of Devcon Two-Ton Epoxy. Mixed it in a plastic cup and applied it with a plastic knife. Wiped off the excess with a little acetone and I was done. We let the toilet sit overnight, did a final clean-up with acetone and the toilet is back in service. The repair is as strong as can be. I did increase the water-level in the toilet to see if the repair was waterproof -- it seems to be but I didn't tempt fate and lowered the level since the toilet flushed fine at the lower level.
So the cost of repair was $2.98 for the epoxy, a little Acetone (women who wear nail polish most likely have this in their bathrooms) and a plastic knife donated by Baja Fresh. As long as the break is fairly clean, this is a repair anyone can make.
I did use regular expoxy rather than 5-minute epoxy. The regular epoxy had a "work time" of 30-minutes so I didn't have to hurry in making final adjustments to the "fit" and in cleaning up the excess.
DJ in VA