Cost is USD 45 including shipping to Oregon in the US. O-ring is made of high strength rubber and designed to withstand the different formulations of today's regular and racing brake fluids. Picture below of the part number.
I might order some new (caliper) pistons in the near future. Circle Track has an excellent article on rebuilding a Wilwood Superlite caliper. You can read about it here.
I also got a new brake fluid reservoir cap from TCEAS. Nothing wrong with the current cap cept very dirty and hard to clean. I know I know.. I am just being anal here..
Picture below of the cap. Cost me a not insignificant RM 80.75 after discount. Made in Japan mah!
Need more (lots of) time to clean up the surrounding area. Can't replace everything :P