As of 01/09/2018, backorder pricing depends on the domain name's extension:
The pricing grid is without taxes.
You are ONLY billed if you secure the domain name.
For a buy-it-now domain name, the price depends on the domain name's quality. This is shown in the domain name's details.
At auction, the final price is that of the winning bid.
Once the domain name has been secured, regardless of how, it will be subject to a monthly subscription charge, which includes renewal fees. The subscription is not subject to a contract. You can cancel it at any time by transferring or deleting your domain name. The price depends only on its extension, regardless of the original price paid.