I'm not sure what you will find for less than $100
I can't think of any portable speakers in that price range which would meet the specs you need. None of these would be giving you much of a bump over your TV sound IMO until you start looking at products such as the Jambox, etc. The original jambox is less than $200, FWIW, if you can still find one. Some of the small ipod speaker docks, etc. really don't cut it for use with HDTVs IME. Very weak/tinny sound, much like the built in speakers.
Some of the Logitech smaller PC/HT surround systems might suffice for around $100. But they are not portable.