When evaluating vinyl windows all corners should be welded vinyl. If the corner uses a metal connector to hold it together it is a cheap window.
Also consider getting an approximate dimension of some of your windows. Take this dimension to your local big box lumber yard and look at their vinyl windows. Get product information, window pricing, and labor costs to install their vinyl window. This will give you a reference point for the windows you're looking at. Any competitors pricing should be relatively close.