Cost of Petrol for one hour drive

We have relatives that are going to pick us up in Exeter and then drive us an hour away.  I have insisted on paying for the petrol.  Can anyone give me a rough idea of the cost.  I know that it varies depending on the fuel economy of the car and on the roads but I'm just trying to get a rough idea.  Are we talking £10, £20, £30 for the one hour one-way trip?   
Thanks

I think £30 is fair. 

I think £20 would be ok too.  We have a small Land Rover and drove a four hour trip last weekend, using almost £50 worth of gas.

I'd go with £20, tops - a tank of petrol for our Nissan Primera (about the size of a late-model Sentra, but with a bigger engine and considered a larger car here in the UK) costs around £45 and we can normally get two 2-3 hour round trips (at 70mph on dual carriageways) plus assorted around-town driving out of one tank.  £20 seems like a fair, round amount that covers not only petrol but their time as well - especially if they've offered to help in the first place.

I'm assuming it's going to be generally motorway driving?  One thing to be aware of if you exceed the speed limit by quite a bit, say 20-30 MPG your fuel mileage will go way down.  If you have a small to medium car you might achieve 35 MPG so if you drove 50 miles you might spend £5-7 for the journey.  I can't see spending anywhere near £20 for the journey unless you have a super gas guzzler but you might want to put in £20 just to ensure you get there with room to spare.

Interesting range. 
 £5-7,   £20,   £30.   

Thanks for the replies. 

If you have a small to medium car you might achieve 35 MPG so if you drove 50 miles you might spend £5-7 for the journey. 

mattj74 is right - there's no way the actual cost of petrol would be anything near £20, unless you spent hours stuck in a traffic jam.  It all depends on whether you're wanting to pay for petrol only - in which case, ask them how much petrol their car used! - or if you're making a thankful gesture to the folks that are picking you up and driving you around, in which case £20 is a nice round amount that says "thanks for your time, petrol and effort." 

mattj74 is right - there's no way the actual cost of petrol would be anything near £20,

I was starting to think my car got amazing gas mileage!