The Dutch paid 7 BN for 1000 square miles.

• A larger project will have a better ratio of dike length to protected area. For a circle, the diameter grows with $sqrt(area)$. On the other hand, people might insist on internal dikes for safety reasons.
• A larger project will have dikes further out, and probably in deeper water.

I don't know how much it will cost, but it will be very expensive, and living in Doggerland would require eternal vigilance to prevent erosion. Much cheaper to irrigate the Sahara.

The Dutch paid 7 BN for 1000 square miles.

• A larger project will have a better ratio of dike length to protected area. For a circle, the diameter grows with $\sqrt{\text{area}}$. On the other hand, people might insist on internal dikes for safety reasons.
• A larger project will have dikes further out, and probably in deeper water.

I don't know how much it will cost, but it will be very expensive, and living in Doggerland would require eternal vigilance to prevent erosion. Much cheaper to irrigate the Sahara.