I offer 100% guarantee on my speaking presentations, not my books or tapes. Books & tapes are ‘damaged goods’ as soon as they’ve been used, so the money I’ve already spent creating them is an expense, whereas my presentations are my time.
I urge you to reconsider this policy. I do understand the point you’re making, but I think you’re looking at the guarantee the wrong way. It’s not about minimising your exposure; it’s about minimising theirs.
If you apply this principle, the actual cost of the goods is irrelevant. Customers don’t care about the cost; they only care about getting value. If they don’t get the value, the guarantee means that they get their money back.