As stated numerous times above, Rebels take up more points and favor quality over quantity, so if "cheapest" is the only parameter then for around $30 at a local store or $20 on ebay you can add either a Falcon, or any 2 of the rebel ships.
Rebel builds with 3 quality pilots can sometimes be respectable.
Right now I'm flying X-wing Wedge + 2 HWKs (Kyle Katarn & Jan Ors). But if you show up at a tournament with that you'll get a lot of weird looks.
Plus it requires the purchase of 3 ships... and at that point you might as well run an XXBB list or something more generic.
Edited by Khyros