Today the low end of the market is all switches. There's not a whole lot of room below $20 to sell something with less functionality. But we thought that the price point for a switch that would do the "right" thing (port mirroring) was too high. (Port mirroring explicitly copies traffic from one port onto another so that you can observe it. And it's your only option if the link is GbE.)
However, Per found a Linksys product with an incredible feature set. The 5-port model (SLM2005) is just $100, and the 24-port model (SLM2024) runs around $350-$400. For $15-20/port you get port mirroring, link aggregation, VLAN support, and a bunch of other features. Very cool to see these features making it into low-end devices.