Don't forget that this includes a trip over the Mersey by ferry as well as the walk through the tunnel, and a classic car show. So all in all, it is actually quite worth the £5.00 ticket price.
A single by ferry alone costs £2.60 nowadays, plus £1.40 to go through the tunnel by car, so that is £4.00 alone. Im not sure if the price includes travel by ferry to and from the event to (as it starts on the Liverpool side) so it may even be more worth it.
Besides which, £5.00 for a possible once in a lifetime achievement, just to be able to say in year's to come, that you did it, like so many can say who did it the few time's it was allowed in the past, is absolutely nothing tbh.
I doubt you will be able to pay on the day; I would think the reason for making it ticket based is so they can account and prepare for health and safety requirements etc... These are the days of New Labour where you can't have a shit in you're own toilet without the HSE wanting to know about it, and Brown trying to tax you on it. Long gone are the good old days where they would have just opened it as a free for all and let anyone and everyone sqeeze through with no mercy!