You get 10 ranks below the rank of the armor itself, for repairs. Say a piece of armor is rank 30, you can repair it at rank 20.
Edited, Jan 7th 2011 6:29am by Spyrit178
I've noticed a couple exceptions to the this rule. For example, I was able to repair a rank 46 sword as a rank 35 blacksmith. But more often than not, the 10 rank rule applies.
To respond to the original poster... I have 5 crafting jobs that are rank 30+ but when I do need repairs or am to lazy to do them myself, I always offer 5k+ (or a few crystals). As a crafter when I'm dealing with strangers, I won't even consider a repair, regardless of level or material, if it isn't 5k+. I think it shows a disrespect for the amount of time and effort crafters put into their jobs to offer such petty amounts for repairs - often below the cost of the repair mat itself.
I remember one guy looking for repairs for an item that required a silver nugget and was offering 500 gil. He also had some shards for sale in his bazaar he was trying to sell for 400 gil each. After spamming shouts for 15 minutes he couldn't seem to figure out why he couldn't get his item repaired. I sent him a tell to say that I'd be happy to repair his item for 500 gil if he wanted to sell me his shards for 40 gil each. He didn't get the hint.
For LS members, friends, and repeat customers, I often offer free repairs.