I dont think its something that really needs a system per-say. As a GM you just need to talk to your players about what they want for there artifact. It should be something that has backstory significace to the char. For instance a warrior might have a sword that has been past down though his family, and it holds great power. Just come up with a base line states for the sword, which should be significatnly better than a reguar one of its same type. +5, or +10 level weapon is what id do for 1 CP. and 2 CP, increasea another +5 or +10. same for 3 CP. You could even give the sword something simlar to a psyonic or magial power that is akin to it. like the ability to spontatniouly erupt into flames worth 3 intensities. Gives the weapon a simple +15 dmg heat bouns.
Mabye iv just GMed a lot of games, but these types of things are balancing decisions GMs have to deal with every adventure. However if your still unsure how powerful, or broken something might be, just remember that you can always start small, and increase its power as the game progresses. The player will be happy, and you wont have put an uber death item of awsomenss into the game off the bat. Whereas the opposite of this, nerfing a power of an item makes the player unhappy even if they understand why your doing it. so start small, and increase as the game goes on until you find a good balance.
If you DO come to a situation where a hero has an item that is to powerful for the game you can always 'borrow' it as the GM. Mabye a sneaky thief stole it when they where not paying attention, or they were knocked out by there arch rival and it was taken to hurt the heros pride. This creates drama, and although the player may hate you for the time being, they will feel a great sence of achomlishment in getting there item back. This is more plausable then all of a sudden having to nerf a power out of the blue. During the time the item is away from the player, it could be damaged, or misused in some form which dimishies its usefullness, which will force the player to find a way to restore it.
A good example is i have a summoer in my group, who took a 1 CP point artifact which is a summoning cube. He also took an -1CP disadvantage, addiction, to the energy the cube emits. The cube has a large amount of presence to store creatures, in additon it has a redistributable 50 point bouns to any of the 4 summoning skills. (he must decide this at the begining of each day / when ever he needs to feed on the energies for his addiction). He was satasfied with this, and if the game was not going to end in a few sessions I would probally find a way for him to increase its power.
Food for thought.