Some schools are a little more specific about the contributions they expect of parents, see here, for eg:http://www.clsg.org.uk/page/?title=bursaries&pid=76
In general, when we looked into it, we found that it's viable for most families only if you have a very low income, or are prepared to put huge constraints on the entire family's life for the sake of one child (which might be fine for an only child, but less so if you have more than one). If you have an average or slightly above average income for a public-sector job (let's say you're a teacher or a bbc producer earning £40k or just above) and you live in the SE of England with associated housing and transport costs it's going to be very hard to make the sums add up. The calculations tend to take equity into account, so if you have significant equity in your house you would be expected to remortgage for eg, which involves a significant risk to the whole family's future financial security.
If you are earning less than about £30K, particularly if there is only one earning parent, or you are a single parent, then the level of support you get might make the sums look more feasible, and if you are on a very low income, at or just above benefit levels, then you'd probably receive a level of support that would cover most or all of the costs.