A pair of Two Pi Bass Reflex divers and Xover will run you $120. This leaves you $280 for cabs. Should be VERY doable for a $400 budget. I honestly don't think you will do much better in you price range, sorry. I would take the advice of hurdy_gurdyamn to heart - look beyond sensitivity. I'm running a 35 wpc PP tube through Maggies (MMG-W, $300/pair), which are about as ineffient as you can get. No, they will not play concert level, but they sound pretty awesome. They do, however, require a sub below 100HZ.
Please let us know what you decide.....Colin