Unless it took basically no effort to build and was thus not something to worry about the investiture of funds and effort in.

 

But that's really an unknown. As we don't know about the "mundane" magical processes. As Harry wouldn't have carred about it Rowling wisely ignored the inner workings of society. Like who builds everything? This is obviously a real job otherwise the Weasleys would have a better house. If a knowledge of transfiguration was all it took then Molly could have been constantly expanding the house as a hobby. The fact she didn't means there's either a set of physical objects needed to start, or the knowledge of how to do so is jealously guarded by the practitioners. It would have to be the latter as the Weasleys have several acres of land, plenty of trees to turn into more rooms.

It would likely be an interesting one-shot to see a day in the life of a builder or other tradesman, but as it has no real interaction with a plot unless Harry was working as a tradesman whats the point?

 

What's wrong with the Weasley's house? It has enough rooms for everyone and won't collapse suddenly without being under attack. A larger place would mean more cleaning, and Molly wouldn't have that!

On the stadium question, it isn't the cost of building it that matters, it's the size it was made. The organizers expected to fill 60,000 seats (and they did), and vendors came to sell them things, and Ministry folk came to arrange transportation and security, so close to 100,000 magicals were involved.

Even if a good portion of the wizarding world were Quidditch fans, they wouldn't all make the journey; now there's closer to 600,000 magicals in the world, though this number isn't as relevant.

Taking into account a differential of distance vs. enthusiasm, and you could say 3/4 of the original 100,000 were locals. That's 75,000 in Britain, all enjoying the idea of being involved with Quidditch, and the Americans won't touch it (sounds like World Cup futbol to me). So of the 75,000 in Britain involved, there are probably twice as many that aren't (that's a very low estimate, btw). Now we're up to 225,000 magicals in the UK. This, I can accept.

A distribution curve of people at various ages (in a society with a near-0 infant mortality rate) bulges at the teen to professional ages. This suggests that 100,000 of the 225,000 in the UK are between 15 and 60. A 45 year span spread evenly across 100,000 people averages over 2200 magical people born each year.

Ergo...

Hogwarts CANNOT be the only source of magical education in Britain. Frighteningly, it seems to be the best and most prominent one.

If you throw that one assumption out (there is no school in the U.K. except for Hogwarts), everything else begins to fit together better.

 

Or that a lot of wizards are home-schooled, or that a lot of muggleborns just aren't picked up on. Either works.