Save your money and get a quality optic and mount. Spend your money once. It only stings once when you buy it. Afterwards you understand why they cost what they do and are damn glad you made the investment, which is what it really is on multiple levels.
My next AR build will be a carbine type. The names I've heard recommended in red dot sights (Aimpoint and Eotech) are a bit rich for my blood at the moment. Can anybody recommend something serviceable for someone who's already sunk most of his budget into guns and ammo without much left for optics?