Depends on the application. Some of the eCommerce sites we run with national audiences, we need a 5 Nine's service. Other companies like a local pest control company, they can usually handle a few hours outage, even an entire day, and it wouldn't affect their business too much.
That said, most of our clients are on higher-end hosting packages costing $30+/month with a few in the $1,500+/month range.
So it depends on the customer, the application, and how long they can afford to be offline. We have one client who's host crashed, and they've been offline for nearly 4 months with no backup. They have no care in the world as they have enough business, so they're building a new site. So for them, reliabilty is not anything they factor in.