As mentioned in the Early Access information section, the $9.99 price tag was always likely to increase as we got closer to final release. With the 1.0 release only a few short months away we will soon be increasing the price to $14.99.
Originally, this was planned to coincide with Main Update 5, a few weeks ago, but it could have come across as a bait and switch to first increase the price only to drop it again for the Steam summer sale.
Don't expect to see the game at such a low price again until well after it's full release.