If the dragon egg is a percentage thing, not a set value, it'll be worth it at higher tiers.This isn't really that complicated to understand lol. He said taxes will inctease 30% a single dragon egg will offset this increase for 1000 souls. Figuring that it takes 15 a day *67 days and suddenly you would have just been better off with the egg.
Additional town levels have higher taxes so the increased taxes will be off set by additional eggs. If you get raided your egg chest can be looted when you lose regions which is an increased incentive to war with other towns as you can essentially steal their investment in eggs to help offset your own towns costs.
Personally I think this will be fantastic as long as the power changes make sieging an actual possibility.