The biggest income I believe comes from increased number of executive boxes. GP has a ridiculously small amount of them. New stadium should also increase matchday sales of other things (refreshments, food, merchandise). And it should improve sponsorship deals. The extra money from the regular seats is really not that much in the grand scheme of things. If you get 10,000 extra seats sold out every week @25 pounds a pop, that's 250,000 per match, or in the range of 5 million per year. With the next TV deal Everton's revenue will be close to 200 million, so it would mean an increase of about 3%, say 4% including all other income from those fans. So while it's a nice thing to have it would mean the club can pay the wages for one star player more.
This is also why financially the gains from a 60k stadium as opposed to a 55k stadium is not very much. It would however increase build costs, possibly require more policing etc. Just to break even would require those extra 5k seats should be sold out for most games for the next 40 years.