Join Date: Jun 2006
Location: west of civilization
Cheapest is the Cosco Scenera, usually retails for around 40-50 dollars. It's a pretty bare-bones convertible, no comfort padding, and no EPS foam.
For a little more you have the Safety First Avenue. It's also a 40 lb seat, but it has a taller shell and higher top harness slots, so she should be able to get at least 3 years rear facing unless she has a heavy kid, and the full forty pounds out of the forward facing harness. That retails for 80-90 at Kmart and Sears.