How do I split the cost of wheels and tires?

I have found a set of tires for the Challenge Corolla. The seller has wheels and tires for sale for a stated price. I could buy the combo and put the tires on the wheels I already have and claim the tires as my exempt set.....OR....I could have the seller split the price of the tires off from the wheels. So my question is how do you make the split percentages? Ideally, all the money for the tires and they come with free wheels, which would end up on the car, but that wouldn't be fair or sporting in my mind. What say you guys? I intend to ask the seller what they think, but haven't contacted them yet.

In for the answer, also wondering how that works.

Ask the seller to write the receipt as tire for $xx and wheels for $xx. The first 4 tires are exempt, so you don't need to worry about them in the accounting, unless they are beyond the 4 free.

In reply to stafford1500 :

I understand that part, but what's a fair way to come up with the relative values of the wheels and tires. My concern is how to fairly value the wheels because the tires will be the exempt set anyway. Finding three ads for the size of wheels is proving to be difficult.