As far as I know, .com and .net have always been the most popular (though reading about .de domains don't surprise me somehow).
I know you can get cheaper domains if you change to a good site for this, but I always have reason to believe that .com and .net will always be higher than the rest. .org too maybe.
I used GoDaddy for my domain and only because they had a sale on another extension. I didn't research any others, but kind of wish that I did. Surely there are prices cheaper than $12+.