In the campaign I run with some of my friends, the fighter in the group asked me if it was possible to have a sword that could turn the damage he dealt into hp, very much like the vampiric touch spell. I looked into it and wasn't able to find any item that fit that description, so I told him I didn't think it was possible. However, we recently had a new person join us who decided she wanted to play as a wizard with an emphasis on magical item creation. As such, I looked into the rules regarding creating your own magical arms and armor and figured out what she would need to do. My problem is that I don't know how much it would cost to make the weapon in question. In the PRD, it gives estimated values for enchanted weapons based on the enchantment and ability bonuses, but because it's not a pre-made ability, I don't know what the bonus would be for the "vampiric" special ability.
My question is: how do I calculate the special ability bonus? Is it just the spell level, or is there some equation that is used to determine it based on other values?