Miliary surplus rifles are cheap and ammo is plentyful.

Mosin Nagant M-44 7.62 x 54 (Big 5 used to sell them under $100.00)

SKS 7.62 x 39 (still shoots ak ammo, but a great gun for the money, can find bulk ammo realitively cheap at local gunshows)

8 mm Mauser (bolt action, accurate and still realitivley cheap)

Remington 700 .308 (accurate and ammo easy to find)

Norinco M1A (Cheaper than springfield armory)

12 gauge pump shotgun (Remington 870, Mossburg defender) Pack a punch for SHTF, can use slugs and buck shot.

Marlin 336 30/30 (cheap gun and ammo is cheap, lever actions are nice too)