Both cost and costed can be used; it depends on the sense in which you use them.
if you use the verb cost as a linking verb to imply to have an amount of money as a price, then it doesn't change in all the tense forms i.e. the present, past, past participle. For example, it costs/cost/has cost
me five dollars.
However, if you use 'cost (out)' as an action verb to mean to determine or estimate how much something will cost you, then you can use costed as past tense and past participle. For examples:
We haven't yet costed (out) the proposal.
The project was costed (out) at $2.5 million.