There are many ways to define "pro" vs "consumer" range, one major point for me is the aperture.
Indeed, many amateurs can get by with a relatively slow lens as the shots they are taking are not either speed or depth of field critical.
Professionals need to be able to cover both, if they are taking action shots that require a fast shutter speed in poor light conditions to earn their living that week then they need a fast lens to help them, depth of field is also very important as it can help make the point of interest in the photo stand out from the background. Or in other words they need as much control over and help from their gear as possible.
I appreciate of course there are those who fall in between, who want to take professional quality pictures, but only for pleasure not reward in which case they are going to buy the right gear for the job.
Basically the faster the lens the more you will have to pay for it, but having said that at least with a DSLR you can change the effective ASA speed without having to change the film, which does at least give you a fighting chance of recording the moment despite bad conditions and a slower lens. You'll just have a noisier picture equivalent to the grain increasing if you move to say a 1600ASA film from a 200ASA one.