As everyone knows - the aim of Amazon, owned by the wealthiest man in the world - is to make money. Unfortunately for sellers, that doesn’t change with lightning deals. The offer of a considerable increase in sales will cost users upwards of $150, and you will have to pay if you wish to run an elongated multi-day lightning deal.
Luckily, the price doesn’t vary on the products sold. Before you get enamoured with the glamour of a spike in sales - consider if you can afford it or if the increased sales of your product will pay for the fee required to access lightning deals.