How much is our sponsorship worth?

I work for quite a small charity (Family Holiday Association), and we are planning a major nation-wide event for next year. It's new territory for us; we've never done anything nearly as big before.

We're quite confident that the event will be a success (a major corporate supporter is involved), and are keen on maximising fundraising via sponsorship. If all goes to plan, the event will comprise a few hundred small events around the country, attended by the general public (~50 people per event).

We have some experience of selling sponsorship (up to £10k value), but I am in the dark about how much this one will be worth.

Does anyone have any ideas to help me get a better idea of value? Anyone out there sell big sponsorship and would be willing to give me a few tips? :)

Thanks!