I did have them replaced about a year ago, didn't do it myself though. The o-rings are about the diameter of a 50-cent piece and are very cheap but the labor was 3-4 hours if I remember correctly. My old o-ring(s) was dry and brittle, like I said earlier the leak only gets worse if this is the source.
2000 E55 AMG
1970 280 SEL (for sell cheap)
1959 Alfa Romeo Giulietta Spider Veloce