I am bit confused. What exactly are you trying to do?
Firstly, if you want customers to have different prices on your web store, then price lists are the best way to do this. It's very easy to assign customers to price lists and will ensure consistency throughout the site and business as a whole. Once it is known which price level to use for a customer, it will automatically be applied throughout the web store. You can set a default price list for your web store as a whole, which is then overridden by what the customer is assigned. Easy.
Secondly, messing with how the frontend price code will just change how prices are displayed. It won't have any effect on what prices the customer is charged, so this is a Very Bad Idea. Just because the frontend code says something is one price will have no effect when it comes to adding an item to the cart at that price as it has to be validated in the backend.
Some price manipulation can be done with Scriptable Cart but, again, I think the best way to do what I think you want to do is to use price lists.