It is difficult to compare, and it will always look lot cheaper to build a new stadium than these expansions, seat for seat, as there are far more constraints.
It also depends what the Everton one included, the cost of the land and the infrastructure around the stadium would bump it up if included. Also either could be well over budget and we may not know for a while.
The most stark comparison is the Millenium and new Wembley, not a massive difference in time but one costing 6 times more than the other for not many more seats, although the spec was very different. Like St Marys costing £32m, different spec, very basic, like most of the Pride Park, Riverside, Sunderland etc basic neo-brutalist stadiums.