When the verb cost means to be priced at or to cause loss or expenditure, it is uninflected in the past tense and as a past participle. For example, we might say that the low-cost milk cost less yesterday than it costs today. But when cost means determine the cost of or set the cost of, it is inflected costed. For example, we might say that the store manager costed the milk at a cheaper price yesterday.
To people in the U.S., costed might sound funny because this sense of the word is not commonly used in American English. Americans don’t have an exact equivalent of costed, but valued, budgeted, priced, paid for, accounted for, and estimated the cost for come close in different uses. This sense of cost is much more common in varieties of English from outside North America, and it is not unheard of in Canadian English.
Here are a few examples of costed used in non-U.S. publications:
The menu is fully costed, reasonably priced and, importantly, returns a healthy profit. [Herald Scotland]
The plan isn’t fully costed and would require hundreds of millions in new revenues. [CBC.ca]
But Mr Abbott said the policy would be fully costed. [Sydney Morning Herald]
[T]he Military Veterans Bill, passed by Parliament last year, broke legislative rules because it could not be costed. [Independent Online]
We find a few scattered instances of costed in edited American news publications, but American writers are much more likely to use other words—for example:
The study authors estimated the cost for these procedures at about $2 million. [Los Angeles Times]
On Valentine’s Day, chef Jerome Bacle will present a four-course menu priced at $80. [Chicago Tribune]
The center was budgeted at $20 million in the current year and $34 million for last year. [Newsday]