What is charged for a normal sized/average website. Very hard to say of course since most projects is different.

Depends whether you use something like Wordpress, I would expect to pay less for something designed on top of a CMS than something custom from scratch. I know our company paid about 3500 GBP for a Wordpress site ( this included backups and management, but I still think we were over charged)

It's pretty impossible to say. I've seen companies pay 6 figures for a Wordpress site and 1k for magento. Then about 5-10 for custom stuff.
My company just charged around 10k for a Wordpress based quoting system that integrates with a CRM. Pricing vary rarely makes sense.

Runs the gamut. Depends on requirements. If the price is too low don't do it. Especially with critical sites.
I have seen this play out hundreds of times. Great price upfront but when you need developer down the road - gone. Either out of business because they prices too low or out of school. Whatever the reason you may be in trouble when it counts most.

Minimum 10. But we do up to 100 of the project is right.