The reason website designers do not post their prices online is because usually website designers develop websites based on what their customers wants/needs. It would be nearly impossible to post prices for website design because of all the different configurations a website can have.
Think of it like this - you go to a car lot and there are 10 Toyotas for sale. Each Toyota will have a different price tag on it, probably not too far off from one another - but the reason is because of these vehicles are different, with different options from the manufacturer.
It is also important to realise that web design is also a service and not just the delivery of a product. With a product, you walk into a shop, the product is there, you pay your hard earned cash and away you go with your new product.
However, in the service industry things are a little different. Have you ever called an electrician, plumber, car mechanic etc and told them you have a problem and want to know how much it'll cost? If you are not specific to the exact problem and solution then it is highly unlikely that they will give you a fixed price without getting details of the problem.
Websites are usually not priced by how many "pages" are included. What if Customer A wants a 10 page website with no special logo design, no graphic design, and no website promotion, just a plain website, and Customer B wants a 10 page website with a new logo for his company, a nice header and footer, and website promotion along with it. Customer A's price is going to be a lot lower than Customer B.