How expensive are blood tests that evaluate liver function while taking medications that may affect the liver?

Not too much: A routine liver function test, if that is what you are asking, is not that expensive. If you are looking for specific drug levels that may be implicated in elevated liver function, that is another story depending on the drug (s) in question.

Call the lab: The costs of lab tests can be widely variable and of course your insurance may pay some part of the fee. Consider calling the lab directly, and asking for the billing department. The two tests you need to know are alanine aminotransferase (alt, formerly called sgpt), and aspartate aminotransferase (ast, formerly called sgot).