The effect and the expense of extending forest rotations for additional carbon storage is investigated. The expenses are clarified in terms of opportunity costs, using methods of financial accounting. The annual compensation needed to finance additional carbon storage can be computed unambiguously. Short extensions are inexpensive, but the unit expenses drastically increase with longer extensions. Including carbon storage in products reduces the unit carbon storage expense at short rotation extensions but increases the expense at long extensions. The unit expense of carbon storage is of magnitude as in boreal forestry, being much less than in tropical cultivations.