But dont you know the 300blk is only tacticool and serves no purpose lol. You wont gain any more velocity out of a longer barrel since all the powder will be burned in 16" or less.
Not true. You will still gain velocity with more barrel length, but gains will be minimal since the 300 BLK is so efficient.
With that said, 16" is as long as I would go on a 300 BLK barrel.
I would look at Ballistic Advantage barrels. They make a line of barrels called the "Modern Series" that are melonited with M4 feed ramps and 1/7" twist. They fit your budget and are good shooters. Great barrels for the money. I have an 8" BA modern series on one of my guns, and it shoots great.
Link to barrel: http://ballisticadvantage.com/16-inch-300-blackout-pistol-length-barrel-modern-series.html