SKS all the way. Unless you reload, this gun should have the cheapest Wally World ammo out of all the others mentioned in this thread. I assume cost of ammo is important to you if you are looking for a semi under $250- it was for me when I bought my SKS. Next to my 22s, it"s the rifle I shoot the most.
Also, if you really want power at a low cost and are willing to go with a bolt gun, look into a Mosin Nagant with a sportster stock. Should be able to put one together for under $250, and the old Commy bulk ammo can be had on the cheap.