why did Unbreakable cost 75m yet Glass only cost 20m?

Because Unbreakable was right after the huge surprise success of The Sixth Sense. So studios were not afraid to throw money at Shyamalan.

Since The Village, Shyamalan has been getting worse and worse so studios are now very cautious.

Most of Glass' budget probably went to Willis, Jackson and McAvoy. That's why the movie is so underwhelming with its setting and scenes.

I agree what the heck did they spend 75 million on? There is nothing in Unbreakable that looks even remotely expensive and all the effects we practical, no cgi.

Unbreakable glass is expensive. Glass is cheap.

Watch here
https://youtu.be/bM1Azo4ieSo

The film was very cheap looking.

I thought it like something a group of friends might film on their phone, except for a few special effects.

Largely, it was a tiny cast running around in parking lots and stuff.

Split was like that too, but it fit the abducted theme.

You can tell a low budget movie if people are mostly in a few rooms or outside a lot. Lots of Netflix movies like that nowadays.