When a slave owner allows a slave to buy back his freedom, he must consider the economic loss to himself. Thus, he may consider the price of manumission to equal the future potential profit that he could make from the labour that the slave provides to him. In this way of thinking, it is clear that $2350 renunciation fee is still pretty cheap. Since the US federal government profits from the US citizen–because in the Internal Revenue Code a US citizen is by definition a taxpayer. It is therefore only right that the US government should try to recover its real costs in losing a taxpayer citizen.
The average taxpayer pays every year about $11,000 in federal taxes, and over the course of 50 year working career that would be a total of $550,000. Of course, if you take the US State Department at its word, the actual cost of processing a US renunciation is $2350, so the true cost of full manumision/renunciation really should be $552,350–but in that light $2350 is merely a rounding error. It may be of interest to know that the citizenship application fees are much less than the renunciation fee (see Why It Costs Immigrants $680 to Apply for Naturalization: Critics argue that the fee is too high, discouraging green card-holders from becoming full-fledged citizens). So I don’t actually believe John Kerry’s State Department when it says that the $2350 fee actually reflects the true cost. Logically, it should cost more bureaucratically to investigate a citizenship application thoroughly than to rubber stamp a citizenship renunciation.