I wouldn't go if they asked me to pay towards the cost of the wedding and certainly not £500 - that's plain ridiculous. Grandious ideas are fine if you have the money, otherwise simplify your plans to suit your means. A good wedding is about family and friends enjoying the day not how many noughts there are when you add up the cost of it.

When we got married we paid for everything and my inlaws thought it was odd not having to pay for drinks - but in my family that was the way we did weddings. I know it's changed now and more often than not money is put behind the bar for the first drink, but that's it everything else is provided.
I paid for the material for the bridesmaids dresses and the mum of one bridesmaid was a great dressmaker and she made them up for nothing for me and they bought their shoes and my OH paid for the flowers.

Jan