Generally the common aftermarket mags are not cheap. Around $40 for the surefire or FBMG mags. I have heard of people modifying the magazines and/or the Saiga to use more common milsurp type mags, for a lot cheaper. This also unfortunately brings 922r into play, and you'd have to change out some Russian parts for US made aftermarket parts to bring it into compliance.
For a Saiga .223, I think you can get Galil magazines to work modifying the magazines only.
For a Saiga .223, I think you can get Galil magazines to work modifying the magazines only.