だったis use a past tense

これはいくらだっけ and これはいくらだったっけ　are grammatically different but make no practical difference in the meaning. Both, after all, mean *"(I don't remember how much it is) how much was it again?" * I am not sure it is a proper translation. Suppose there is a book with the price of 1000 yen and you forget the price, then which should you say, "これはいくらだっけ" or "これはいくらだったっけ". Yes, the latter one refers to the past thing, but still makes no difference. Because the price of the book is always 1000 yen now, today, yesterday, a month ago, 3 years ago, tomorrow, and month later. Whichever question you use, the answer will be the same, "1000円だよ“ あれ、明日、どこに行くんだっけ？ and あれ、昨日、どこに行ったんだっけ？ are different grammatically and semantically. In the former, the speaker is wondering about tomorrow's destination and uses a grammar for the future（行く） while in the latter, wondering about where they have been yesterday, and uses the grammar for the past（行った）. The answer will be different for each question. If I mention a subtle difference between これはいくらだっけ and これはいくらだったっけ. The former implies これはいくら（で買ったん）だったっけ　or simpler これはいくら（で買ったん）だっけ. He was wondering at what price he (or others) bought the book.