Archiving costs a minimum monthly charge of $2.99 for up to 25 GB of storage. Once you exceed 25 GB, you will be charged $.05 per additional GB. Archiving is billed monthly for the previous month's usage.
If you're in the Unlimited plan, archiving is included in your plan. You can archive as many photos as you want, at no additional cost.
When you remove all galleries from archiving, you will still incur one more bill since archiving is billed retroactively for the previous month when the service was used.