Well, talking about ONE ad only here.
Depending of how many declensions (not sure is the right word) will come (i.e. A3, A2, portrait landscape, brochures, etc) might be thought in advance, even before planning the layout , this is extra work. But IF it's a simple ad, standalone, the charge depends on the customer as well, this is where your knowledge about his budget should help you. If you ask too less, you lose some credit, questions about your professionalism will rise (it happens), if you ask much too much will scary them away.
But i'd say the risk to lose a client is lesser when you ask above average than below. Once they liked your previous work they ready to negotiate. If you ask low, there will be no negotiations for sure
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Enough with theory, price can vary. Most of them are in range between 150-1500usd (not talking about very special campaign and special clients), and not mandatory the expensive ones were most artistic, more complicated or more time consuming.
I'd say the price factor depends like 80% on the client (and country).
Try to charge them as you charged other for similar work when you were pleased with the budget for the job and raise it with 20%, at first meeting.
Wish you good luck selling yourself well
Not sure I helped you somehow, but , morally, I'm with you