You are better off not going to a motorcycle shop - they sell motorcycle helmets which have a different spec to motorsport helmets. Some organizations do not permit motorcycle "M" helmets as they have a lower level of fire retardation ability than special application "SA" helmets. You should look/ask for a helmet with a "SA2010" sticker on the inside of the helmet, stuck on the shell underneath the padding. Motorcycle helmets will have a "M2010" sticker.
Oh, and a more expensive helmet may not necessarily be a safer one. You pay mostly for more advanced materials and widgets that make them lighter and more comfortable to wear. As long as they have a Snell certification, they have passed the minimum test standard to be acceptable for motorsports use. However, there will be models that exceed the minimum spec too!