Is she telling the truth, or is she lying?
This is the acceptable one, if I have to answer you your question as it is, Gustavo.
And technically, you are asking just one question, it looks like you are asking two questions, though.
It´s called an "either-or question'', you have to choose/judge/decide if she is telling the truth or lying about the actual price. Are
they telling the truth or is
this the false price?
Comparing the question to Portuguese questions (with verb to be), and (for explanation´s sake) "splitting the two questions" [as per your own description ]
, it would be an "inverted version (as they say in Brazil.)
So, "are they telling the truth?" and "is this the false price?' in both the -to be- verb comes first. When they are in the same sentence they (the questions) will follow same the pattern as well.