When it comes to buying upholstered furniture, it is just like buying a sports car...you might want a red convertible, but there is a big difference between a Hyundai, a Toyota and a Ferrari.
You are in the market for a sofa, and it is shopping time! Stop on it at the furniture store that promises more for less, and you see a lovely linen loveseat for $599. Then, head on down the high end retail furniture store, they have one there for you at member's price for $3500. Call your designer friend, she can sell you one for 6k. BUT THEY ALL LOOK THE SAME!!!
Don't judge based on the fabric-the fabric is the smoke and mirrors. So, don't think about it until you know whats under the hood!
Evaluate the cushions, there are three main types: Foam($), Envelope($$) and Spring($$$)
Foam ($): Can range in qualities, firmer foam will generally last longer, but won't be as comfortable, softer foam breaks down faster
Envelope ($$): Will have the same firm or soft foam inside, but is surrounded by a removable envelope of down and feathers or a polyester down blend.
Spring Down ($$$): contains an interior pocket with springs for long lasting comfort, provides the cushion without the breakdown that straight foam can come with. The foam surrounding the springs is encased in an envelope of down and feathers or a polyester down blend.
How can you tell?
Unzip the cushions. If you find batting, you have foam. If you find a cotton, second cover, either spring down or envelope. To tell the difference between those two unzip the second cover and feel the center of the cushion for the springs (you will have to judge carefully, that part is difficult to gauge, unless you know what to look for.