I planted 5 shallot bulbs that I bought from the store. In return, I just harvested 30 bulbs. About half of my harvested bulbs are the same size or larger than the ones I bought. About half are smaller. Given that it was a poor year for onions in general, I think that's pretty decent. They didn't use up much space in the garden. I haven't tasted them yet but they'll be on the menu soon!
I also tried growing shallots from seed. I started seeds in the basement and transplanted the seedlings outside. Those are still growing. I'm getting one good-sized bulb from each seedling. It seems more economical to grow them from seed because there are lots of seeds in a packet, and a packet costs less than buying the bulbs. But it requires a lot more work. A trade-off.