remember a 10" layer cake can be cut in half-in both directions and give you 4 times as many charm (5") squares- sometimes buying the layer cake makes more sense cost wise- you can cut it in half- then save one half- 5"x10" and cut the other in 1/2 again and have 2 x's the charm squares.
i seldom purchase pre-cuts- if i do it is only because i do want one of each fabric in the line- and i choose which ever pre-cut is the most (cost effective)- whether it be a layer cake- a charm pack or a jelly roll- then i purchase yardage of coordinates to go with it- but only if i just have to have one of every fabric-and there is more than 20 of them.