As a general contractor for the last 29 years, I have experienced many times where potential customers would call and ask for a construction estimate. I would go to their home, talk to them of what they wanted and then they would ask for an estimate. I would tell them that I can only give a "guesstimate" because there are no formal plans to bid off of. They would undoubtedly contact additional contractors and maybe sign a contact without plans for their project. This is where the gamble comes in as they open themselves up to expensive change orders for items that may not have been clear.
The money saving smart way is to first contact a designer such as our self's and have a site visit to give a general construction cost. But more importantly, your project would be on a plan that multiple contractors can bid upon and obtain an apples to apples estimate.
One last note on this.. You will also find that contractors will be more interested in your project and return calls if they know you have a set of plans.
For more information please see our website at www.SRbuildingplans.com