DS9 revolves around a unique trade route between the Alpha and Gamma quadrants. The route was an artificial wormhole built and controlled by the Prophets. At the wormhole's accidental discovery Sisko was inside making some negotiations while the Cardassians were also vying for control. It seems that the Prophets have no needs, they don't even experience the passage of time. What did we give them in exchange for free access?
KIRA: The Sisko taught us that for you, what was, can never be again.
AKOREM: If the D'jarras belong in the past, why did you send me into the future?
ODO: For the Sisko.
SISKO: For me?
AKOREM: You're saying that he's your Emissary?
BASHIR: He is the Sisko.
AKOREM: Then I've been wrong about everything.
OPAKA: Why? Why do you stay here?
SISKO: Because I still have questions.
OPAKA: We are of Bajor.
SISKO: What does that mean?
OPAKA: You are of Bajor.
The Prophets want Sisko there for Bajor. As long as he is there, they are content to allow use of the wormhole. They prevent Sisko from resigning (Far Beyond the Stars) and keep Sisko from killing himself to stop the Dominion invasion (Sacrifice of Angels)
There is never any indication that the Federation gave anything at all "in exchange for" passage.
The pilot suggested that the passage of ships was somehow harmful to the Prophets, but by the end of the episode we are told:
Station log, Commander Benjamin Sisko, stardate 46393.1. The lifeforms who created the wormhole have agreed to allow safe passage for all ships traveling to the Gamma Quadrant.
This agreement is never mentioned again nor discussed in more detail.
So, something may have been given back to the Prophets in exchange. But, if it was, we have never heard anything about it.
Personally I interpret the phrasing of Sisko's log entry to suggest that nothing of value was given in exchange, that the Prophets simply decided to allow ships to pass through having "got to know" Sisko a little during the course of the pilot.
Perhaps the "harmful" nature of passage through the temple had been theretofore imagined: a general concern about the unknown ramifications of unchecked infiltration by these aggressive, adversarial time-aware alien beings!
And besides, as others have noted, it was ultimately in the Prophets' best interests to do this, so perhaps that drove their actions in the pilot in some way.
The Prophets didn't receive anything in trade from the Federation or any other organization in exchange for use of the wormhole. They seemed to allow free passage to anyone who wished to use it. Though there was one notable exception.
You could make the argument that allowing the Federation to use the wormhole gave them frequent access to their Emissary, but that's not the same as trade.
Also, the wormhole is naturally occurring, not artificial.