Short answer: materials (the glass itself and hardware) and labor are usually split close to evenly on most jobs, with custom shapes or premium finishes shifting more of the cost toward materials.
Materials
Glass thickness, tempering or lamination, and hardware finish are the main material cost drivers — a straightforward rectangular panel in standard chrome hardware sits at the affordable end of this range.
Labor
Precision measuring, custom cutting, and careful installation (especially frameless work with tight tolerances) make up the labor side — this is where quotes vary the most between companies.
What a transparent quote looks like
A trustworthy quote itemizes glass, hardware, and labor separately, rather than bundling everything into one number you can't evaluate against a second quote.