If you extend a stadium to get 10k extra bums on seats Villa would have to pay what 80 to 100 million.
The rest of the stadium is perfectly serviceable. So we'll take the higher figure for ease of math sake, cost per seat £10000 given ticket prices and extra revenues perhaps they break even in 8-10 years given interest.
For us to get to that point we need to knock down and upgrade at least 3 stands and half a dozen streets to do it properly. 300 million, although it won't be as by the time you do one it will be 5 to 10 years so all the costs will increase. Our 30k per seat cost will end up with us spending large amounts of our income to pay it off for decades. Our tickets are cheaper and each stand will only bring marginal gains as it will be 13k instead of 10k etc. as you go through the build. We'd run out of money by the time we'd have finished two stands.
The whole point of the new stadium is on day one you have the whole increased extra capacity all paying more than in the previous stadium, eating/drinking more, better hospitality that people might want to pay in a better area and you get the stadiun sponsorship which you would never get from Goodison. One practically pays for itself due to the extra income generated and the other would mean out of our merge income we would have to find the full 300 million plus interest. It isn't rocket science.