"Please cost both" is an inelegant way of asking for prices of both the products. A better way to ask is "Please (give me), prices for both". The reason is that cost and price are not synonyms and have distinctly different meanings.
The verb 'to cost (something)', as an intransitive verb (as used in the example), means to calculate, to assess, or to estimate the total cost of a product (for example, to determine the selling price of a manufactured product or the landed cost of a purchased product). Example: 'In view of increased raw material costs, he costed the product xyz again'. (Note: 'costed', here, is right).
The verb 'to cost (someone)', as a transitive verb, means the amount of money that someone needed to incur or pay. Example: 'The foreign trip had cost him a packet' (Note: 'costed', here, is wrong).
The verb 'to price' means arrive at an amount as price (to sell / transfer). Example: 'The increased costing has forced us to price the product higher'.