There is the Sig 551 and there is the Sig 551-A1.
There are very few actual Sig 551s stateside and they have always been priced way north of $2000.
The A1 was Sig's response to the people who weren't satisfied with the 556 Classic and clamored for a real 551. The 551-A1 was made in the US and takes original 551 mags that rock in like AK mags but does NOT have the same proprietary barrel/bolt assembly and is of generally lesser quality.
They were sold exclusively through CDNN and priced around $1600-$1800 IIRC.