They were from X-mas 1999. The information is
here
They came in the form of a deed and once you double clicked it, it would give you a random potted plant.
You can only buy them from vendors now and yes, they do cost quite a bit.
I'll give you some prices that I've typically seen them sell for. The potted tree sells for the most.
Potted Tree: up to 2 million.
Potted Flower: 1 - 1.5 million
Potted Fox Glove: 1 million
All the rest: anywhere from 500k to 1 million.