They are alot cleaper than they were even 3 months ago. i got 2 for $225... which seems like alot, but 2 months ago guys were selling for $150 each PLUS shipping. As I said, mine seat and feed 15 perfect. I don't think the follower would change the legality of the mag. The follower is just a piece, like the spring. The mag is not modified. If that were true, putting a magpul follower in a 10 round pistol mag would be against the rules.
At the end of the day, can ANYONE point to ANY authority who would A: be in a position to be checking you for this and B: Be in any way educated enough to know or tell the difference. IMO, they are going to crap a brick when they see your mag can kill a whole flock of babies long before they call you out on a follower. Once you explain, prove, defend, demand and finally get your mags back... Im %100 sure no one will have mentioned the follower. not once.