Oi Freddy, he's in CA, so that changes things.
Two main affordable options for you:
SKS: 7.62x39mm, dead-reliable and easy to work on if you so choose. Most actual stores in CA charge silly prices for them, so you're best off buying one online and having it shipped to a CA FFL dealer. Go to
www.gunsamerica.com and use their "FFL Finder" to find the best price for FFL in your area.
Bear in mind that the most common cheap SKS these days (Yugo 59/66) have grenade launchers, so aren't CA legal. You want to find a Yugo 59, or any of the Romanian/Albanian/Russian/Chinese-Norinco SKS out there. Russians are a bit pricey, but you should be able to find a CA-legal SKS for under $200 on GunBroker, AuctionArms, GunsAmerica, etc. Tack on $15 for shipping and $25 for FFL transfer, but you don't have to pay sales tax on it.
Kel-Tec SU-16CA: Run a THR Search on this model, lots of reviews written up. I've seen them as low as $475 or so in SoCal last year. CA-legal, but accepts AR-15 magazines, .223 caliber. Vaguely a combination of AR and AK concepts put into a CA-legal package. I'd skip the A and B models and go right to the CA model.
That's about the size of it until you get up to around $1000 or so, unless you want to count the M1 Carbine, or semi-auto hunting rifles. Good luck in your search,
-MV