The answer to this question can be found fairly early on in the story. Readers are told that the initial setting is inside a "Time Safari" office, and then we are told that Eckels walks in and waves his check in the air for the world to see. It is a check written for ten thousand dollars.
"Warm liquid gathered in Eckels' throat. He swallowed and pushed it down. The muscles around his mouth formed a smile as he put his hand slowly out upon the air. In that hand he waved a check for ten thousand dollars to the man behind the desk."
That's a ton of money for a single ticket. That might not sound like that much money in today's terms, but Ray Bradbury wrote this story in the early 1950's. $10,000 in 1950 is equivalent to over $104,000 today. Regarding this particular story, the amount of cost associated with the trip is not what is important. What's more important is how the check helps characterize Eckels. He doesn't humbly submit his check. He pompously flouts the fact that he has that kind of money. He thinks that his money somehow makes him more worthy of respect and honor; however, by the end of the story, Eckels is shown to be a complete coward.