Before the ATF ended the import of 7N6 because of some foolish companies in the US making and importing 5.45x39 pistols, 7N6 could be found for 16 cents or less per round. Now, the cheapest I can find 7N6 is 21 cents per round. New manufacture Russian non-corrosive steel case FMJ ammo that doesn't have steel cores in the bullet is 23 cents per round. Additionally, the Hornady steel case loaded with a 60 gr VMAX is one of the only (if not the only) factory loaded 5.45x39 ammo that's suitable for hunting and better for defensive use than FMJ.
In comparison, 7.62x39 FMJ from Tula is 18 cents per round, and Wolf is 20 cents per round. For hunting or defensive use Remington, Winchester, and Federal all load JSP, Cor-Bon offers JSP and Barnes DPX, and Hornady offers a SST.
Also, 5.56 NATO is 26 cents per round for Tula or Wolf steel cased FMJ, and there's a huge selection of factory loaded hunting and defense ammo in .223 Rem and 5.56 NATO.
Finally, there's the price of the rifles. New, US made AR-15s are going for as low as $500 + shipping. The cheapest I can find a new, unconverted Saiga in 5.45x39 is $500 + shipping. A new US made Century AK-74 is $610 plus shipping. The next lowest priced AK-74 I could find is the Waffen Works, and that's a used parts kit assembled with a new US chrome lined bbl and other compliance parts, and it's still $620 + shipping.
In other words, a $500 Saiga or a $600 AK-74 shooting 21 cent per round corrosive surplus in a caliber with only one factory loaded hunting round isn't really cost effective anymore. When you could get a Saiga for $250, spend about another $100 for parts to convert it yourself, and get surplus ammo for 13 to 16 cents per round it was such a great deal that it didn't matter that ammo was corrosive, there was no hunting ammo available, and you'd voided the warranty by converting the rifle. Now, more accurate AR-15s like S&W M&P-15 Sport ready to go out of the box with a lifetime warranty are available at prices as low as $550 shipped. Non-corrosive .223 ammo only 3 cents more per round than non-corrosive 5.45x39 (or the same .223 for 5 cents more per round than corrosive 5.45) and there's that huge selection of hunting and defensive ammo already mentioned. So, with prices what they are now, the base model AR-15s are a better choice for many.