You can either subscribe or buy PT 12. Subscription licenses stop working when you stop paying. Buying gets you a perpetual license that will work, well, in perpetuity :)
https://shop.avid.com
Buy:
Pro Tools 12 with 1 year free upgrades:$599
1 year support and bonus plugins: $99 (optional)
Subscription:
Pro Tools 12 yearly (includes support and bonus plugins):$299
Pro Tools 12 monthly:$29.99
Keep in mind you will also need an iLok 2, and any plugins you currently use in LE will need to have AAX versions to work with PT 12 (that might be free or there may be a cost, depends on the plug).
I think I have the above info correct, hopefully someone will double check :)
Joel