The first thing that I should mention is that embryo adoption is not technically considered adoption, but rather a transfer of property. Though their are embryo adoption agencies that charge money, it would be fair to say that embryo adoption is generally of a not for profit nature, and that the costs involved can certainly not be compared to that of private domestic or international adoptions. That does not mean that the cost of embryo adoption is insignificant, though. If you are interested in adopting an embryo from another family, there are several ways to go about finding a good match for you.
The cost depends on the route that you decide to use. I think it is safe to say that embryo adoptions organized by fertility clinics are often the cheapest. Reproductive endocrinologists are sometimes asked to make sure that a couples' left over embryos are placed somewhere, often at no additional cost. Don't be afraid to ask about this option. If you successfully adopt an embryo or several embryos in this manner, the costs that will remain are the costs of fertility medication and transfer of embryos into your uterus. Private embryo adoption agencies are, however, a different story. One embryo adoption agency I looked at states that the total cost of embryo adoption ranges from $6,000 to $17,000, and uses the fact that this is cheaper than newborn adoptions as a selling point. The agency costs were said to be from $2,500 to $6,000.