Its true the lowest price is usually either from someone who cuts corners or is desperate for the work. Just as you should never go with the highest price because you think it'll be the best quality, you shouldn't always assume the lowest price will always be the worst job.
In my experience its important to gather three or more quotes to get a ballpark idea. What is more important is to look for recommendations from people you trust. Once you have identified/narrowed it down to a couple of front runners you get off your arse and go and view thier work, speak to their previous customers and get an idea of the experience they had dealing with that company. On a project that size only then would I pick a contractor.
Going with the cheapest quote and thinking 'job done' is just asking for trouble