E.,
We are also fans of Britax for safey reasons. When we needed to buy a second set for our nanny's car (2 more seats -- we have twins), I bought the Fisher Price Voyager seats. They are made by Britax, but sold under the Fisher Price brand name. Much cheaper, but still good quality.
I would also recommend www.albeebaby.com, as someone already mentioned, if you want to buy the car seats new. I have purchased many items from them (4 car seats, 1 stroller), and have gotten great deals.
You can get the Fisher Price Voyager car sets on there now for $110: http://www.albeebaby.com/fiprsavodeco1.html
Check under their closeout sections as well (http://www.albeebaby.com/closeoutitems.html), often you can get less popular patterns and/or last years model at a great price. I think they have some Britax on sale now too.
Hope that helps - good luck.