In the most common case, the past and part participle of "cost" are both simply "cost." "Today, it costs $189.95. Yesterday, it cost $269.95. If it had cost $189.95 yesterday, I would have bought it yesterday. Perhaps it will cost even less on Black Friday." Your third example is interesting. The grammar is correct, but there is a logical problem in the real world. The grammar expresses a continuing or progressive action in the past so it is not logically correct. Suppose it has cost you $1,000 each month (end of January to end of August, perhaps), and the total cost has now added up to a total of $7,000. Your grammar is correct for the continuing, recurring monthly cost, but you would need to say "$1,000 per month." Thus, these two sentences are both grammatically and logically correct: "It has been costing me $1,000 per month since January." "It has cost me $7,000 since January." There is a rarer form of the verb "to cost" that is used only in business situations. It means "to determine the cost of." In this case the part participle is "costed." "We are having this problem because we failed to cost the shipping charges realistically. If we had costed them properly, we would have realized that we needed to price the contract higher. In future, we will cost them more conservatively and include seasonal adjustments."

Why do you ask questions that can be answered by looking in a dictionary? New Oxford American Dictionary cost | kɔst | verb (past and past participle cost) [with object] 1 (of an object or action) require the payment of (a specified sum of money) before it can be acquired or done: each issue of the magazine costs $2.25. • cause the loss of: driving at more than double the speed limit cost the woman her driving license. • informal be expensive for (someone): if you want to own an island, it'll cost you. 2 (past and past participle costed) estimate the price of: it is their job to plan and cost a media schedule for the campaign.

cost/cost/cost. There is no 'costed' :)