I'm surprised nobody has answered with the absolute cheapest way to go for 308 power.
Join the Mosin-Nagant club. Ammo is not as cheap this year as it was last year, but it's still in the 50-80 bucks for 440 rounds.
You can go with the M44 which is more carbine length.. or the 91/30 variety, which once corked, and pre-supposing you find one with good rifle grooves, will easily shoot 1 moa. Mine was picked up for 100 bucks and had been drilled for an ATI mount. I slapped on a Wally-Mart scope and instantly became a "Vasily Zeitsev" sniper. All of this would come in under your budget, and you have in your hands one of the most historically recognizable rifles.
Just be verrry careful. I don't know anyone that has purchased just one! They are infectous.
KKKKFL