I did teh 5.45 thing a few years ago, when mags were $3, and ammo was $200 per case.
However, even today, it'll depend on how much you shoot. If you only shoot 300 rounds a year, it's not worth it, but after buying a 5.45 wasr, and 10 mags, shoot 5-6K rounds compared to even 7.62x39, and...