Since it is a .380, I'll assume that it's a commercial mak with adjustable sites. I'll also assume that it takes standard 8 round capacity mags (some of the .380 commercial maks high-cap 12 round capacity guns). With the original mags & box and all...I'd say it's worth $200. TOPS. I too have seen them offered for a lot more...but I don't think people are really paying $300. for them.
here's one in 380
http://www.gunbroker.com/Auction/ViewItem.aspx?Item=153847091
and there are no bids on it, whereas this one has bids (but it's a 9X18) right about where you said
http://www.gunbroker.com/Auction/ViewItem.aspx?Item=153333460
and mine looks like this one as you described, adjustable rear sight, (2) 8 round mags, no holster. It has the Pearce rubber grip on it now, and I have the crappy plastic grip for it somewhere I think.