This may be a mistake.

[v0031]

Though Tomahawks have been in service for years, newer versions have been equipped with better communication systems, among other things.
Each unit cost about $569,000 in 1999 dollars, according to the US Navy, -- equivalent to about $832,000 today.


I think it should be "costs".

[This account no longer in use.]

In this case, the grammar works; "cost" is in past tense.