You could just have the customization be an additional item, this way it would retain the same price for the customization regardless of how many the user wants to buy. If you would like to have it like this then you would need to define how you would like your user to choose this customization option. Probably one of the easiest ways would be to have a checkbox and use that as a trigger for a second add to cart server behavior. So if the user checks the box then you have an add to cart server behavior that will occur and set the price in the cart as well as any other aspects of the customization you would like to include.
Please post back with any questions that you have about any part of this and we will be glad to help get you going.