Blanket statements on this subject are hard to make with confidence.
One of the unique aspects of the business profile of the modern prostitute is that the supply and demand elements combine closely with the factor of her financial needs to determine price.
Now I would not make a habit of phoning escort ads in the paper that say $200 and 'bidding' $150 and hoping I am the lucky caller to whom she says "yes, yes, a thousand times yes".
But when out driving the streets it doesn't seem unreasonable to offer a price first, without knowing what the woman tends to ask for when she tips her hand initially.
No question there is lots of bartering going on out there on the streets of Vancouver, as there can't be many other reasons why so many will drive up so close and beckon a woman to chat through the open passenger window only to drive off with no transaction being made.
Why would guys invite the police to swoop down upon them that way without gaining anything from the interaction, unless it was bartering about price? (they are clearly "communicating for...")
Depending upon the time of night or time of the month (rent-wise, check-wise) a working girl on the streets may well adjust her prices, so it doesn't seem unreasonable that a guy might negotiate just a bit out there.
If he just got $120 from the cash machine and the woman he pulls up to says: "$150", it seems well within what is fair for him to say that he only has, and can only spend $120.
IF that is true on the street, then I think it blurs the lines a bit in those ads in the 'Straight that might state "$200". Some like to use the word "...firm" after the price, and that isn't a reference to body type.
Perhaps by omitting the "firm" reference (as in "$200 firm"), a person unknowingly opens the door to bids within reason.
Even the world's oldest profession is evolving somewhat with the times. Compare this with the hotel industry, where the only establishments who once had access to the blurry-eyed, late-night traveler on a budget were the small motels. In this day and age one need only make a reservation on Priceline to put the bigger, chain hotels in touch with the low-budget little guy. Those big hotels don't want him because of his immense prowess for shelling out the bucks. They know the room will sit empty, with zero revenue, if they don't invite him in on his terms.
Having stated all of that, I simply cannot remember the last time I tried to negotiate down with a working girl. Part of that comes from sort of knowing the going rates in the areas where I seek their company. What is right for men who are brand new to this business and who will be somewhat 'surprised' by almost any price a street worker might name?
I agree with those who don't generally want the service reduction likely to come from a woman whose going rate has been negotiated downward, yet I wouldn't agree that some customers who do this, in some circumstances, are always to be frowned upon.