The price depends on the actual model, so the cheapest 64GB version is down to €339/$319/£279. The 256GB gets a bigger discount and is selling at €439/$419/£369, while the 512GB model goes for €539/$519/£459.

Here is what Valve says about these refurbished units:

Certified Refurbished
Each Certified Refurbished Steam Deck has been thoroughly tested to the same high standards as our retail units. Every device goes through a complete factory reset, software update, and an extensive examination involving over 100 tests at one of Valve's facilities. Among the tests are all controller inputs, the audio system, the screen, and internals. Battery health is also assessed to ensure proper functionality and longevity.

All refurbished units meet or even exceed the performance standards of new retail units. Although they may have minor cosmetic blemishes, they provide a reliable, high-quality gaming experience at a lower cost.

All Certified Refurbished Steam Decks include:
The same one year warranty as a new Steam Deck (unless longer as determined by your country of residence)
A refurbished power supply - fully tested (may have minor cosmetic marks)
Carrying case and Quick Start guide

You can check them out over at Valve's dedicated page.