To test this, we have temporarily set it so that there is only one product referral (and such that Google cannot show any other products).
Every conversion of this product is supposed to pay a non-zero amount (it follows both from the description of the product, and from the definition of “conversion” in FAQ).
When you go to Advanced Reports, and get the Referrals history, the daily earnings are expected to be the number of conversions times the referral price. I noticed recently, that this has not been always the case. The daily earnings are always a multiple of the single conversion price, but it is often given by this formula:
Earnings = (number_of_conversions – integer_number)*conversion_price
This integer_number is usually less than 1/3 of the number of conversions. This is very easy to see when you have only one product, but if you have many, the formula would be obfuscated due to different conversion prices. I suggest somebody tries this out. Since Adsense ads and Referrals probably share a lot of common code, I would not be surprised if the same bug reduces Adsense ads earnings as well.