In the case of a keyboard you would not need more than 4 layers even for a fairly dense ball grid chip because you have lots and lots of space (the keys require spacing) and that lets you run traces fairly circuitously. But a 4 layer PCB is about 2.5x the cost of a two layer board in the designs I've done, and still the cost of the PCB is quite small. (One design I did a two layer version was 0.85 per PCB (4 x 3.5") and when I pushed it to four layers to get down to under 4 x 2" it was $2/PCB so 2.35x the cost.)