There is a time tested rule that says you can sell 60% of your customers an upsell that costs 60% or less of the original purchase price. The upsell must be congruent with the original purchase. In other words, don't upsell a gardening book to someone who just purchased a weight loss book.
Bottom line: We often speak of testing different price points for the main product, but you should also test your upsell prices, too. If you can sell your upsell to 60% of your customers simply by lowering the price to 60% of the product purchase price, it might well be worth it.
With that in mind... how many Internet Marketers in the IM niche are getting this right?