how much does the rift pass cost ? and if I buy it will all the rewards that I have locked become unlocked ?
Rift pass is about 1080 Auric Cells if I remember correctly. You will unlock what you have already unlocked I believe.
It's 1000 auric cells, which is about 10 dollars/euros worth. And yes, if you buy the Rift Pass, you will immediately receive the premium rewards from every tier you have reached so far.
It's 1k Auric Cells, as said above, to unlock the Premium Pass. If you already got to Tier 70 on the Free Pass, than all the Premium Pass rewards will be unlocked if you buy the Premium Pass afterwards.
It's important to note that every Premium Pass will give you the full 1k Auric Cells back if you complete all 70 Tiers of The Rift. Doing so will give you your 1k Auric Cells for future use on another pass AND all the rewards that Rift gives. So, if you're skeptical about the rewards that The current Rift gives, than you can get to Tier 70 (or a random Tier) and decide for yourself if it's worth paying the 1k Auric Cells for. If you naturally get to Tier 70 for every Rift, than you'll be rewarded all The Rift rewards for that Rift AND the 1k Auric Cells back as a side bonus.