A major contributor to the tone quality, strength, and price of an acoustic guitar is it's construction. This inside acoustic guitar diagram illustrates the internal parts of a high quality acoustic guitar.
The interior parts and how they affect the guitars tone, are explained below the illustration.
It takes a great deal of time and skill to brace an acoustic guitar this well, so, cheaper beginner guitars are not made this well.
Guitar bracing serves two functions ... it strengthens the guitar, and the size, shape, and angles of the bracing can be used to "color" the tone.
Cheap beginner guitars use pieces of scrap wood usually pine or another low quality softwood.
Top quality acoustic guitars primarily use high density spruce, with the grain aligned vertically. The grain is vertical because the wood is "quarter sawn", which makes it much more difficult to break than weak wood with no particular grain alignment.
The high strength to weight ratio means that the braces can be "scalloped" as you'll notice in the acoustic guitar interior diagram. The lighter weight scalloped braces allow the soundboard to vibrate more freely, which gives the guitar a fuller, warmer, more resonant tone.
There are more factors that contribute to an acoustic guitars sound, read more in the section here: How to Choose an Acoustic Guitar
The braces in low quality guitars are there only for strength ... and because of their grain orientation do a poor job of it.
I'll add a wood grain orientation diagram soon.
Definitions of the Guitar's Interior Parts
X-Brace: This is the main brace which forms an X pattern just below the sound hole. The degree of the X shape contributes to the balance between treble and bass tones ... a lesser degree raises the treble, and a wider splay increases the bass.
Soundhole: The sound hole's size also contributes to the tone. The larger the sound hole the more treble.
Neck Block: Sometimes called the top block. This is where the neck fits into, either by tenons or screws. Tenons are usually more conducive to sound transmit-ion.
Tail Block: sometimes called the bottom block. The main purpose is for strength at the bottom of the guitar, and for tail pin support.
Lining: The lining's main purpose is to widen the gluing area for the attachment of the soundboard and the back to the sides. Most lining has many small saw cuts to facilitate bending and is called kerfed lining.
Tone Bars: They strengthen the lower section of the guitar body(lower bout), and to balance the treble and bass tones.
Treble Braces: These strengthen the top and increase the treble.
Cross Strut: The cross strut strengthens the top under the fretboard. It's thick due to the high tension the neck exerts on the top in this area.
Soundboard: This is the main vibrating plate of an acoustic guitar, and produces most of the sound. Top quality soundboards are quartersawn, which makes them stronger so they can be thinned more for better resonance.
Bridge Plate: This strengthens the area under the bridge, and provides a hard surface so the ball ends of the strings don't wear through the relatively soft top. Cheap beginner guitars sometimes use softwood, which doesn't hold up well, or plywood.
Side Bracing: These are usually only found on guitars with sides tend to crack such as Brazilian Rosewood.