BTW a lot of us will never forgive Cheaper Than Dirt because of their actions following Sandy Hook, they instantly jacked up their prices to gouging levels, and kept them there for a very long time. They also pulled their guns from sale like a bunch of wusses. I and many other vowed to never forget how they spat on the second amendment and their customers.
If it is for long term storage I would look for the lacquer sealed ammo like Golden Tiger (Good luck) Klimovsk or Brown Bear. I would buy it in spam cans of 640 or 700 rounds. If you just plan to burn it, poly coated Wolf is dirty but runs my rifles just fine. I have had exceptional luck with the ammunition store and SG ammo. Those are two very good outfits. If you want off the shelf Academy Sports Monarch is usually lacquer and under $5.00 per 20 rounds which is as good a price as you will find. They sell hotshot Romanian too, not bad.
In Canuckville this is a favourite round. For 280 bucks you can get an SKS and 660 rounds. Just have a small bottle of windex and a patch ready after some range time to neutralize the corrosive primers and it will last a lifetime. For that price every Canuck shooter has one to be certain and ammo galore (some of us have 2 or more) My fav is the old Chinese surplus made for the Russians it's very consistent and I've never had a fail to fire. Ever. Not sure if you can get the Norinco in the US right now but that would be my recommendation. 80 bucks for 660 spam can is good value for a reliable round. Got a bag of old frying pans just sitting here waiting for a nice day to hit the 75 yard range on the farm and practice with iron sites. I realize M4 is a more favoured platform but there are a couple of after market stocks and mag options that while they are a little bulkier make for a still very usable setup.
I'd check out all the above and look for price. A lot of it is made at the same places but each company charges differently. For the record, I do personally avoid venders I consider unscrupulous regardless of price.
It ticks me off that you Canucks can get Russian Tula SKS refurbs in like new condition from West Rifle and a bunch of ammo for $280.00, That's like $5.00 American! Oh well, I guess I'll just have to be happy with my $300.00 beat up battlefield scarred type 56.........................and my 30 round magazines.
I use SGAmmo. They always have the best, or close enough that it doesn't matter. Good service. Been using them for years, they didn't jack up their prices in late 2012, early 2013... They just went on what seemed like a month long family vacation when they sold out of every round.