I see bid and ask quotes for stock market index futures (e.g. the S&P500 mini future contract). However, I've just read from multiple sources that there is no cost to enter into a futures contract. If so, then what is the bid/ask supposed to reflect - is it the strike price of the future, or is it the price you will pay for entering into a contract with a certain strike price?
Another way to formulate the question: Is there only one exchange-traded futures contract for each index with a fixed strike price, or are there multiple futures, a futures chain like there is one for options?