Disneys Wonderful World of Knowledge Books worth

I'm just wondering if you can tell me how much my complete collection of Disney The Wonderful World Of Knowledge 1999 would be worth? They're in really good condition apart from a few minor dents on the spine of some/edges and I think there's a kid's name written on one of them. If I was to sell them today how much would they be worth? And if i hold on to them for another 20/30 years, how much would they be worth then?