I have the same problem in that I want it to look as good as I possibly can. I previously used jewellers gold bar tags that are discrete and just wrap around the clip and stick to itself with glue that doesn't stick to the pen. Unfortunately you have to write onto the tag and didn't look that good. I also have the problem that I keep records of all the pens and on every pen I give a business card with details of the pen, what its made from, kit etc. so also have an identifiying number which didn't give me a lot of space so I didn't put the price on these labels but on the card only. This meant that people had to ask the price which is good in one way, the fact that they're not frightened off by the price and once talking to you, the sales process starts.
Not liking these tabs, I changed to craft labels but these are cumbersome and can easily come off the clip.