Hi Carolina,
... the way we would go about determining a fair market price for a sketch is this: (1) write in your notes about the sought for sketch- (a)the general subject, (b) the medium [i.e. pencil, wash, etc.] (c) the dimensions of the sketch and (d) the quality of the ground [i.e. newsprint, cardboard, illustration board, high rag content sketch paper, frame -if any-, etc.] (e)the artist [i.e. famous name or unknown or student, etc.]
Then (2) with notes in hand visit several local L.A. art galleries, looking for sketches that match your wants. (3) when you find a work which is a good approximation of your sought after art, write down the price asked by the local gallery. Then DEDUCT from that price the probable cost resulting from middle-man handling, taxes, duties, shipping costs and retail profit (say, at least 15% additional for profit). FINALLY, you should have a price you know is fair market price. When in Paris, your plan should be to obtain your art for a price a few points below what you now believe is a fair market value in Paris.
... Don't hesitate to ask L.A. art professionals (including art dealers) questions along the way. Their experience is valuable. Happy Shopping! P