4 mags plus night sites and LNIB as it sounds, $400.00 would be a steal, $430.00 - $450.00 would be fair both sides.
You didn't mention whether or not was Gen 3 or 4, but the prices on used ones are within fifty bucks, so a little higher for a gen 4 vs. 3, however unless one is left handed, the gen 4 in a baby glock doesn't offer a whole lot unless you want to add to the grips to make them larger.
The recoil spring is the same on the gen 4 and gen 3 in the baby glocks.
This auction on gun broker just ended last week for a used Gen 3 - 27 with night sights for $420.00. It looks to be in very good condition.
Used Glocks are not commanding as much of a price as they were 8 months ago, especially in 40 S&W. The nines still sell fairly quickly, but look at COMPLETED auctions for the past month on gun broker in glock 27 to see current market prices for pistols that actually sold!
Remember to factor in possible shipping and FFL transfer costs into the equation and/or sales tax if a new Glock within state when figuring out your bottom line price to buy or sell.
Saw one for $449 at a gun show recently. It had Night Sights and two mags. It was pristine and it was a used firearm. New Gen 4 started around $525 with no extras, with most going around $549. The Gen 4 comes with extra mags. There was one with night sights for $599. Those were the only ones I remember looking at. The price would depend on whether you are buying or selling and to whom you'd be selling. The prices I listed were retail and not good enough of an incentive for me to consider a purchase. I might be able to do better by shopping around, but a Glock 27 was not on my shopping list that day.