It actually depends on which level of customer you are marketing.
If you are aiming for the low price budget end of the market, then putting prices on your website and van and anywhere else, takes out the time consuming part of weeding out customers.
Therefore only the people who are going to pay your prices get in touch, and therefore most if not all your calls are converted to work.
If you are aiming at the higher end of the market ... then as Simon says keep them off... 
Which begs the question 'why on earth would any business person only want to do business with people that only ever go with the lowest bidder?'
Especially as there as many, if not more people willing to pay a higher price in exchange for great customer service and fabulous quality.The extraordinary thing about this, and the reason it makes no business sense at all, is that the lower price guy pays the same for his chemicals, equipment, vehicles, fuel, insurance and everything else it takes to run a business as the guy charging higher prices, so the only thing that can suffer is his level of profit, which is bound to be less.
The problem is that a lot of people coming into this business believe that being competitive is about price, when in actual fact it is and always has been about quality. I dare say lots of the low price guys do a great job, so why not charge for it? The answer to that is they can't let go of that core belief that the only reason people come to him is because he is the cheapest.
I have the good fortune of knowing some seriously successful carpet cleaners, people who practically print money and it is not because they are the cheapest, far from it, but for one reason, they have succeeded in putting their customers in a position where they can't get what they do anywhere else and made their customers see them as, 'their carpet cleaner,' and that for anyone who is interested is the Holy Grail of carpet cleaning.
There are people like Susan and Julian Dean who have a particular business model based on low prices, the difference between them and the guy that knows no better is that they have chosen that model and made it work for them very successfully using a TM, as I am sure have others.
Simon