The cost of a medium session depends on the platform, experience, and popularity of the medium since it relies on the same criteria as a general psychic. As a result, you can expect to pay more or less than the average rate depending on your personal preferences. On credible psychic sites, you can find more standard rates across the board for mediumship readings.
There are usually valid reasons why some mediums charge more. For example, you can expect higher costs with a trance medium that allows spirits to use them as a vessel. Since trance mediumship is more specialized than regular mediumship, it makes sense for the cost to be higher.