When the game first updates it will enter an early access period, and will be $10 for the first 5 days, then $5 for the next 5 days and then $1 until the next update is released.
Once the next update has been release, the previous version will then become free.
Please be aware that if you pay for early access to an update, you're not paying a one off fee for life time early access to all future build versions, you're only paying for early access to that build version. If you want early access to future build versions then you'll need to pay again.
If you do not wish to pay for early access, then you do not need to as the game will be free to download after the next version is released.