Is the canvas used in pre-made packs the same as plain canvas pieces?
No, they are the same. But there are A LOT of different ready made canvases to choose from, prices ranging from pennies a square inch to tens of dollars per square inch. Price depends very much on materials and construction that went into building the pre-stretched canvas. 

Most important thing to know is what your own needs are as an artist. If you like to paint on burlap then buy burlap canvas. If you like the feel of Belgian linen buy Belgian linen. Once you know what your needs are then seek out what will satisfy that need and buy it at the best possible price.

Get to know the artist you are.

Get to know the prices people will be charging you to be that artist.

Here's a list of the things I like to look at when buying store bought ready made pre-stretched canvases.


The frame the canvas is sitting on, what material is it made from, how are the corner joints made (are they fixed or free floating), are there cross braces, lap joints on the cross braces? Learn the language of carpentry.1
What is the material stretched across the frame, Burlap, Cotton or Linen? Is this a material I like to paint on? What's the weave pattern and thread count? Did I like the results of my work the last time I painted on this fabric? Learn how to identify fabric weaves.2
How was the canvas stretched and fastened? Are the staples (or tacks) on the back (gallery wrap 3) or on the sides of the frame? Are the staples stainless or galvanized? 
How was the surface sealed and primed? How many layers of primer was put on? What was the primer? Do I like painting on this type of primer 4
Is the canvas on sale and do I have good experience with it? When I bough this canvas the last time did I end up liking the results? What did I pay the last time? And what is the price today? Am I getting screwed or is this a good price? 5.


It's a huge draw back buying something you don't like, at a price you could have gotten cheaper elsewhere, that meets none of your likes or needs.