I have a few simple (Wordpress or simpler) PHP/MySQL sites that get a couple thousand pageviews/mo apiece. They don't need more than ~99.98% uptime. I spend $5/mo apiece to host them on a shared host (asmallorange.com, if you care). I could probably find similar featured hosting for half that, but my experience is that the extra $30/yr buys you good customer service.
At work, our site serves 8 figures per month in page views and targets 99.999% uptime. We're on a Python/Django/MySQL stack with Celery, Varnish, Elasticsearch, and a few other things thrown in for fun. Between staging and production environments, and redundancy in production, I'd estimate we spend around $7k/mo on AWS for hosting, plus about $3k/mo for a couple Redshift instances for internal use. This year we might reserve some of our instances to save money.