If bursaries are availble, it's usually mentioned in the 'fees' section of the website. If not, a quick call to the school will get you the answer.
There's no way to give an answer to "how much would I need to pay" as each school sets its own income/asset limits, and each has a different amount in the pot for new bursaries each year. Some might prefer to give one large one, others several smaller ones, and it might vary each year depending on the candidates.
You need to work out how much you can afford, and then you will see what size award you need. When you go to look around the school/s on your short list, you also need to make an appointment to see the bursar and find it if they might, in principle, to be able to offer an award that size.
You also need to find out what they are looking for in all 7+ candidates, and what, if anything, else in bursary candidates. Bursaries are offered to prospective pupils they really want - some are looking for academic prowess, others enthusiastic all-rounders.