Saiga in 7.62x39
First you get a brand new Russian built AK47 type rifle for the price many SKS rifles are selling for currently.
Then you spend the $100 or so it takes to make it a standard AK rifle, with the ability to use standard AK mags.
Spend the remaining money on magazines and ammunition. Or. If you are feeling crazy pick up a 308 Saiga as well.