500mb is quite a bit. Have you considered using wifi when available? In my experience there's quite a bit of wifi just about everywhere and I rarely have to use my roaming plan on trips abroad.
To answer your question, I don't know of any decent-priced plans that offer 500mb per day in other countries than the home country. I can point you to the plan I use though, which I'm satisfied with. It is one of the better deals available for roaming in the Netherlands.
Vodafone Netherlands offers pay-as-you-go 'contracts' called BloX. Two of these are aimed at roaming. There's a small bundle and a bigger bundle. They are daily 'contracts' to use abroad. Nearly all European countries are included. And if you cross a border that day, you don't have to pay twice. For 2 euro per day you have 35mb internet and for 4 euro per day you have 100mb.
Sadly, the website is only available in Dutch, but that may not be a problem for you, judging from your location. Anyway, here's a link to the pay-as-you-go sim cards.