I like the idea of adding relationship and risk vs reward modifiers, but I'm not too sure about adding level and wisdom modifiers. Do NPCs get more contrary as they increase in level? Being wise somehow makes it more difficult to get that person to agree to a good deal?
Level should factor into risk vs reward. At higher levels, certain rewards become less effective. A +1 weapon may be valuable to a 4th-level fighter, but is worth relatively little to a 20th-level fighter with a +5 Vorpal weapon. On the other hand, as NPCs get more personally and socially powerful, risks are also reduced.
Wisdom should seldom enter the picture, unless it is a case of short-term reward vs long-term benefit or danger. To give an extreme example, consider the classic case of an attractive female trying to lure a guard away from his post. A guard with high Wisdom might see that any short term benefit he might enjoy is not worth the risk of leaving his post.
