As a response to the US adoption of the 5.56x45 cartridge, the USSR researched and decided to go with a high velocity, small caliber cartridge also. The Russians decided to neck a 7.62x39 case down to 5.45 and chambered it in the new AK-74, which was a modest redesign of the AKM.
Although, the US's 5.56 cartridge and the USSR's 5.45 have a similar bullet size, they are nowhere near interchangeable.
Currently, AK-74 accessories are less expensive than that for AKM/AK-47 rifles. Classic Arms [(800) 383-8011] has 4 AK-74 (5.45x39) magazines in an East German pouch for $19.95 as opposed to $32 for AKM (7.62x39). Ammo is only slightly more expensive at $129.95/1000 for 5.45 as opposed to $99.5/1000 for 7.62.
Out of curiosity, what AR-15 appearing rifle shoots 5.45x39?