So the question is what is a fair price to pay for an event?
Surely the answer is a price that covers the cost of the event.
If the question is 'what is an acceptable price to pay for an event?' then we can get into the rights and wrong of making profit, covering costs of accommodation / travel (perfectly fair, why not?), covering the cost of your lunch, etc, etc.
Look at it this way. If you volunteer for an organisation as a volunteer that organisation has the right to reimburse you reasonable costs (travel, lunch, child care, etc) if it so wishes. So you are volunteering your time to help (in this case a game) why should you not be afforded the same (is you so want it) without being made to feel as though you are taking the mick and exploiting the community in some way? Or should you only be able to run events if you can cope with a bit of a loss at the end of it?
So I suppose as a TO you really need to make a decision of what your boundaries are and ultimately if you can afford to be out of pocket due to unexpected costs, if not why not try to make a surplus and put this to the next event?
I can honestly say I've never been to an event where I have thought the TO is taking the mick in any way, shape or form with the price / value. If I did end up going to one I would either think fair play, you've put your time into it and deserve a little bit of reward or, if the price way way off, wow you're taking the mick I'm never going to one of your events again (in which case their profit making event organising career would be short lived IMO)
And the answer to the original question is.....around £10 (more if food is provided)