This is something I really hate about domains, they have to be renewed or I will loose them. I own around 50 domains and some of them are good, but I also have bad ones, even very bad ones. For the bad ones it is easy to decide, I will let these expire and drop. But for the names that are between good and bad, it is very hard to decide.
The renewal costs are not cheap. If I have to renew 10 names, it will cost me around $ 70 in total. I already renewed 3 domains, including my top hosting domain. Domains left to be renewed are 25. 20 of these names are .eu names. They will expire soon and I decided to keep 5 of them.
A smart thing to do when you are going to renew some domains is to check google (or another search engine) for coupons. For almost every well known domain registrar you can find coupons and you get a discount.