I hear good things about Dell switches (I think they're rebranded HP or something, but still...). Their managed switches start around $400 retail. Stay away from Extreme. I've heard very evil things about those.
I doubt you're going to find any
managed 100Mbit switches for less than $100, even on eBay. There are plenty of old Bay Networks 10Mbit switches, or 100Mbit managed hubs flying around...
Just try searching eBay for "managed switch" and see what you come up with.
Edit: Here's the cheapest 100Mbit/managed that I found Dell 3024 managed switch