The total-price calculation
Multiply the decimal price of each selection. Four matches at 1.50 give 1.50 × 1.50 × 1.50 × 1.50, that is 5.06. On a stake of 1,000 units the total return would be 5,062. That is the part bookmakers show in big letters while you are building the ticket.
The calculation nobody does
Turn each price into a probability and multiply those instead. Four selections at 1.50 imply two thirds each, and the product lands near 20 per cent. In other words: four times out of five the ticket dies. A parlay is not four bets, it is a single, far harder bet.
Why bookmakers promote them so hard
Because the margin multiplies as well. Every selection carries its own built-in commission, and combining them makes the total margin grow with each leg you add. A six-leg parlay is the most profitable product in a sportsbook's catalogue, which is exactly why it sits on the front page.
When a parlay does make sense
When each leg, on its own, already looked like a value bet. Combining two plays you would have made separately anyway is acceptable. Combining six to reach a pretty price is trading a decision for a raffle, and paying extra margin for the privilege.
The variants: systems and cash out
Systems (3-from-4 and friends) spread the risk across several smaller parlays and cost more up front. Cash out lets you close early, always at a price worse than the mathematically fair one. Both are useful, but both are paid for: they are not free shortcuts.
How we look at it
We publish single-leg plays with a minimum price and a stake, because a single bet can be evaluated and a six-leg parlay cannot. If you are going to combine anyway, run the total-probability calculation before you hit the button.