There is a small sterling silver pen that fits the small pen loops (the ones that on some agendas close the agenda, and on others, are inside). They go for about $190, I believe. Be aware tht the flared top of the pen will mark the inside of your agenda if it closes firmly (with a zipper, or against other things in your bag or briefcase).
If you can get a cheaper, thin ballpoint I would personally go with that.
I have one of those pricey slender sterling H. pens, jtroops, and I've had nothing but trouble with it. After about two uses, it wouldn't retract any longer, and I gave up on it and returned it to Hermes - the replacement didn't work any better.
I'd take mistikat's sage advice and look for a less expensive pen that will fit and will actually work. (I found one at Sam Flax Art Supply. It came with a small agenda, which I had to buy, but I think the total cost was about $10, and I now have a pen that actually works, fits in the loops and won't damage the agenda or anything else in my bag, including the bag!)
I have the sterling silver pencil. I personally like it a lot and have had no problems. The SA recommended it to me instead of the pen since the pen can mark and in case there is an ink accident your agenda isn't ruined.
I also have the silver pencil and I never had any problems with it. It's a dupont pencil, I assume their pens are the same. Personally I would never carry pens after having one exploding and ruining my jacket one hot summer some years ago.
I purchased a wonderful little pen for 6.99 that fits into the loops. I will go and find the name. I purchased on eBay. I found it doing a search to see what other folks were using since I did not want to spend $$$ for the Hermes pen.
It is called the Zebra T-3 mini and it comes in a pen or a pencil. I bought it from Winningpen who was a wonderful seller. He has them for sale right now on Ebay. My pen came with 2 ink refills. Only has blue ink.