Ask, too, if they'll help you find a service provider and register a company domain name. If someone at your company will be helping design the site, establish up front how involved that person will be with the outside designers. If your person can write in HTML, ask the designers if they're available just to organize the site, with the in-house person doing the actual creation. If there are several designers you like, consider paying each of them to create mock-ups for comparison. These mock-ups should look like little flow charts, showing how visitors would progress from one page to the next, and where links to outside pages might be.
Designers price themselves many different ways.
Some charge by the project, some by the hour, and some by the page. Because it's a new industry, there's no pricing standardization yet. The cheapest designers are those without much experience, willing to take on their first clients at a discount rate in return for finished products to list on their resumes. Many small companies gamble on untried designers. But some get lucky.