There is an ongoing discussion on the warranties board started by a former employee whose protection plan is essentially worthless now that he is no longer employed with the company. The problem is that the plan only covers up to the original purchase price, and now that he no longer gets an employee discount... that amount is not enough to cover a full replacement of the defective product.
Similarly, customers who buy a protection plan on sale items are going to face the same problem... a voucher or credit for the sale price will rarely be enough to replace the defective item with an identical or comparable replacement.
Long story short, it is impossible for the current protection plan (which is based on "purchase price") to make customers whole on sale items.
Why not just assess the cost of purchasing the BTPP based on the regular pricing of that item?