What do you mean by "not too expensive"?
If you want good ships, remember to make clusters of weapons, each with their own factory/reactor/storage. A single shield (if you have one) should cover all of the weapons in the group. Then spread these clusters out until the destruction of one won't destroy the rest of your ship.
To get lots of firepower, just to put large cannons everywhere. They're already armored and have great DPS. A single LC is 6k and a factory (in 0.15.15 i think) is also 6k.
Just look in ships designs if you want ships.
