I've found the rules on crafting magic items and making existing ones rather contradictory. For instance, in the Pathfinder Core book, the Cloak of the Montebank has a base value of 10080gp, however if you use the rules on making magic items, doing the same thing would be 12900gp? I've used the same formula with the Horn of Blasting and came dead on, and yet looking at some like the Harp of Charming, Horn of Blasting, etc... have been thousands of gold cheaper than they should by using the creation rules. Most of my issues seem to come from the wondrous item section. There are some items that myself and my players would like to create but we're all flaberghasted by the rules. Anyone have any good insight or clarification to these rules that could help us out?
Those aren't rules. They are guidelines and a starting point. When creating a new magic item, you are supposed to start with the guidelines and end with "comparing it to existing items." Also, if you are not the GM, have him involved in every step of the process.