Ive bought the 40 grain American Eagle 5.7 for as little as $13 for a box of 50 from PSA. They frequently have sales. The biggest problem with the 5.7x28 is the lack of decent ammo availability. FN has a lock on anything more exciting than FMJ and wont sell the "armor piercing" ( its not ) or the subsonic etc. Its not quite a pain in the ass to reload but the brass is kind of weird due to the way the FN PS90's operate with a shellac coating on the casing. As far as the usual pricing its not that bad. $15-$20 for 50 rounds. FN ammo being higher but no more than $25 unless you just like overpaying.
The reason I do not own the FN is that if the mag is not in the gun, it won't fire. Will the 57 fire without the mag in it?