"Cost (base verb), cost (simple past), cost (past participle)" comes under the category of irregular verbs. Other verbs that are like "cost" include: "cut/cut/cut," "put/put/put," and "read/read/read" (but the pronunciation changes for "read").
Sometimes "costed" is used informally by people who have overgeneralized the "-ed" suffix.
Accountants may use "costed" and "costed out" (only in the past tense). My Webster's dictionary says "costed" means "to estimate or set the cost of."
Example from InvestorWords.com: "By tradition, a company's accounts are usually prepared on the historic(al) cost principle, i.e. that assets are COSTED at their purchase price."