You can do some bargaining with the salesperson. They typically work on commission, so will usually sacrifice a little something to get the sale. A good thing to ask to be thrown in is a walking foot for the machine.
Be sure to bring your own fabrics to try machines out. Take your time and try out several brands, models, etc. If this is a machine you want to use for both piecing and quilting, also bring a practice quilt sandwich to test free motion quilting.
I personally prefer machines that are quiet; loudness or clunkiness indicates to me less precision in the parts.
Don't feel obliged to make a decision on the spot. Get the salesperson's card and tell them you want to think about it. Also, be aware that many dealers will not take back a machine if you change your mind. It's very important to be sure of whichever machine you eventually take home. If that means trying out machines at a variety of dealers, take time enough to do that.
If the machine you choose has many features you are not accustomed to, be sure you are close enough to take lessons at the dealership to learn how to use your machine to its full potential.
Also, ask about used machines while you are there. Many people trade in a perfectly good machine for the newest model. Often you can get a better machine used than you can for the same money new.