What I have found is that often the machine is the same but the other things that are included in the purchase can be different. It is just like buying a computer from one big box or another. The difference in the prices is what programs and extras come with it.
At least with the computers there is a letter code included with the model to indicate the build configuration. When buying on line I want a list of everything in the box. If the purchase involve programs be sure to verify that everything is exactly the same then go for the best pricing.As far as the included lessons go be sure to find out what they plan to teach you for that extra $$$. $600 sounds like an awful lot for a lesson on how to thread the machine. Any dealer that is wanting to sell me their machine better be prepared to spend some time with me at the demo machine showing me how to thread it and use the machine.