AimSurplus.com has German 147gr FMJ 200 round battle packs for $80 = .40 cents/round
CheaperThanDirt.com 140gr soft points 20 round box for $8.19 = .41 cents/round
You're buying a good rifle. Get good optics for it, too. Remember, cheap surplus ammo is fine for plinking. The full potential will come from shooting quality ammo (match ammo). Most of these ammo will have a Boat Tail bullet design. They are usually listed as FMJ-BT or BTHP or listed as "Match Grade".
ATI ammo is a nice boat-tail ammo that gives good accuracy. .64 cents/round
In bulk, this
DAG ammo is not a bad option. Better than most bulk surplus. .43 cents/round
Ammo generally does NOT go down in value, and it keeps for a long time. If you find a good deal, BUY lots! You can always sell it later and get your money back and/or make a profit!
Gunbroker.Com is a great source for some good deals. It's like eBay but for gun-related stuff.
Lastly, consider reloading! It's a $250-$400 upstart, but you can break even in a few years and save a TON of money in the long run. I've been reloading handgun ammo for 15 years and I've made my money back in the first 2 years, and been saving over 50% moving forward!