I think the term "mini helicopter" refers to 400 and 450 size electric helicopters, which are in the range of 24" to 26" long. I doubt you'll find a RTF in that size for under $100.

Maybe what you're looking for is "micro" heli. Size range 200 to 300 electric, which are aprox 14" to 20" long.

Then there is smaller than that, I think called "ultra micro". Size 6" to 12". Hope this helps.