When does a developer need to pay Apple? (App on App Store)

OK.. so to look at it another way:

I develop an app and offer in-app purchases. Apple takes 30% of all in-app purchase - yes?

But if my app redirects people to an external source to make payment, would it still be eligible for that 30%?

Is that clearer? I fear I may have worded my initial question very badly! It's been a long day!

Well this is why i'm confused - because the service we want to provide doesn't require the need for "In-App" purchases in the sense that most people think of In-App purchases. The idea is that I open the app, compare deals and prices on something and then when I click on the "book now" for example - it takes me to the actual site to make payment. The transactions are not handled by the app - the app is purely a resource. But as I understand, any app that has in-app payments are subject to the 70/30 split with apple - but as we are not directly taking payment - I wanted to know whether that was still applicable - but it would now seem that it's not possible to develop an app that redirects a customer to make payment - is that correct?

Thanks all. I was just trying to find out if this was even feasible. I wouldn't start working on it until I knew for certain - and I had planned to speak to Apple directly as well - but as the Apple community is so helpful, I thought i'd start here first - I figured someone may have had experience!

That's what I was looking for - we're not offering goods in app - they are real world purchases, and i've now found that you cannot do that using Apples In-App API - but can be offered using external payment gateways - which is what we would be doing.