For measuring the purchasing performance you need to compare the purchasing costs against another value. The problems come to define this 'another value'. Unfortunately, the worst solution is the most popular. Usually the top management compares the purchasing costs against the sales prices following this calculation rule
Purchasing Performance % = (Purchasing Costs / Sales Prices ) x 100
Doing that, top management is not really measuring the purchasing performance but some kind of 'purchasing weight' in the sales prices. This value can be interesting but this is not purchasing performance.
If your sales guy is really incompetent, or detest the purchasing department as usual, he can decide to reduce the Sales Prices, so the percentage of Purchasing Costs will increase, and the 'Purchasing Performance' will decrease.
Let's see how to find a method to work for everybody : sales, purchasing and top management.
In my opinion, there are 4 different problems.
1) Customers managed by Sales department and Suppliers managed by Purchasing department are in general working in two different markets with different rules, cycles and ways to work.
2) Top Management doesn't care about problem number 1. They are focused in profits.
3) The main difference between sales and purchasing, regarding the way to work is that purchasing department buys items and sales department sales final products. It can be a stupid difference but it is the root cause of a lot of misunderstandings.
4) Top Management doesn't care about problem number 3. They are focused in profits.
So, It is time to propose a new approach.
To be continued.