There are two possibilities.
Please tell me how much I should pay for this.
Please tell me, how much should I pay for this?
In the first sentence, how much I should pay for this is the direct object of tell. In the second, please tell me is simply a polite way of introducing a question.
Thus the second sentence is spoken with question intonation, whereas the first one isn't.
Edit. There is some criticism of my answer in the comments. It appears to be directed not at the substance of the answer (what sentences are acceptable) but at the fact that I have identified the clause how much I should pay for this as the direct object of tell.
It may be that linguists' views have changed in recent decades on whether it is appropriate to consider that a clause can function as a "direct object" in English. I will note, however, that the grammar by Quirk (Comprehensive Grammar of the English Language, p. 1050), which was authoritative when it was published in 1985, has this to say.
Subordinate wh-interrogative clauses occur in the whole range of functions available to the nominal that-clause [...]:
direct object: I can't imagine what they want with your address. [...]