http://www.apld.com/apldmembers/index.asp
Prices vary, but you can expect most to charge $45-$95 per hour. Some firms only design, some firms are design-build companies.
A landscaping company is NOT the same thing, most have no design based education; and sadly, many designers have only scant horticultural knowledge.
Free estimates are almost unheard of among professional landscape designers; usually the ones who offer any services for free are those who are associated with nurseries, and the design is free if you buy all the materials from that nursery. Free estimates are common, however, among landscape contractors, but most of them look like the ones on the BGH website: they are not creative, they are not based on the architecture of the house in ANY way, and they are overloaded with expensive "gee whiz" features like illuminated waterfalls.