Sofas-$599-5,999....What's the Difference?
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 $3500. Call your designer friend, she can sell you one for 6k. BUT THEY ALL LOOK THE SAME!!!
SO...LETS DECONSTRUCT!
Fabric:
Don't judge based on the fabric-the fabric is the smoke and mirrors. So, don't think about it until you know whats under the hood!
Cushions:
Evaluate the cushions, there are three main types: Foam($), 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.

Frame:
Ask about the frame...Again...3 basic options, here: Plywood ($) or Solid Wood ($$) or Solid, kiln dried, hardwood ($$$)
Plywood ($): lightweight and inexpensive, this material is the cornerstone for a cheap sofa. over time, the wood can warp. When relocating the sofa, it can get tweaked and will become flimsy feeling.
Solid Wood ($$): Moving up in the world! Here, at least we are dealing with a construction material that can handle some wear and tear. However, if it is not kiln dried and hard wood, it can still warp and crack. Hard wood is slower growing and more dense, than something like Pine (which is a typical 'solid wood' frame material)
Solid, Kiln Dried, Hardwood ($$$): This sofa will be heavy, dense with structure and framing, and wood that has been grown slowly and will last you a lifetime. Kiln dried, means, the wood will not warp, as it is already nearly 100% dry.
How can you tell?
Lift up the end of the sofa...If it feels like you can flip it over, you are likely dealing with plywood, if it feels like you are pushing yourself at the gym, likely solid wood. But, if you have trouble getting it off the ground...then you likely have a piece constructed of solid, kiln dried hardwood.
Frame Construction:
For this section, we will only discuss the high end, proper way to support the joints of a sofa. The corners should be blocked, doweled and glued. It is possible that solid wood ($$) sofas share this characteristic, so make sure to ask! This is vital to the longevity of the piece, as well as it's durability during transport.
How can you tell?
If you have determined it is plywood, don't bother. If you are dealing with solid hardwood or wood, ask a sales rep and/or sit on the sofa and bounce a bit. You should not hear squeaking. Lift up one corner of side of one end of the sofa, it should not squeak or warp, at all.
Foundation:
Sofa's have foundations, just like the foundation mattress of your bed set!What type of foundation are you dealing with?
Webbing ($): essentially, polyester straps stretched across the frame. These straps are not resilient, offering no cushion, and if they are, they will stretch and sag over time-making your cushion sag
Sinuous Wire Springs ($$): these are zig zag shaped wires that create an base for the cushion to sit on, they are relatively durable, offer some flex and cushion, but can sag over time.
8-way Hand Tied ($$$): this foundation is a base of individual springs that are tied together (in 8 different directions) to create a durable and comfortable foundation for the cushions. This is considered the top of the line, for good reason.
How can you tell?
Pull the cushions off. You should be able to feel what is under the decking fabric that conceals the foundation. See the pictures below to know
what you are looking for.

I would never encourage buying throw away furniture. My philosophy is to buy the highest quality you can afford and buy it for the long haul. I'll let you in on a little secret...the most comfortable piece of furniture in my house is a Baker sectional I purchased on Craigslist. At our stage in life, with three young boys and a host of projects around the home to save for, the idea of spending over 10,000 dollars for a sofa that was a quality that would last was out of the question. My options were to buy a cheap one....(not on your life!) sit on the floor (did that for a while) or get shopping creatively (done!) We now have a 4 piece sectional with spring down cushions, 8-way hand tied foundation, kiln dried, corner blocked frame, in a fabric that I don't mind seeing dirty feet on, and an extra 9k in the bank!