An expired ENS name will first go into a grace period. After that time, anyone will be able to register the name again. Once the name has left the grace period, it will go into a dutch auction starting with a premium price currently set at $100k.
Over 28 days, the premium price will decline to have no premium. In this way, a user can register a newly released ENS name by choosing a time to buy during the premium auction window. The current price decay is linear. For example, purchasing a name on the 14th day will have a premium price of around $50k. The price reduction is updated frequency, so it's not static and will continue to decline.
In the user interface, the ENS name in a premium auction period will indicate a required premium fee. Clicking on the chart will give a price estimate on the decay line. You will need enough Ethereum to pay the premium fee plus the registration fee per year.
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With the increase in the popularity of ENS names, names available to be re-registered would be instantly bought by bots, and front-running miners, to snipe an ENS name before humans could purchase it.
The current starting price for the premium auction is $1,000,000, which decays to $0 over the 28 days. This declining premium is effectively a Dutch Auction.
We do not intend for users to purchase a name at such a high price, but it prevents those running scripts from taking advantage of the auction mechanics.
For more information and a discussion on the current price premium, see the post outlining [EP 9] on our governance forum.