I think I could help answer this. The cost of computation has gone down so much that its just a fraction of the price for your whole genome sequencing. The cost comes from preparation, reagents, and storage. Illumina sells their own reagents, so the cost is really dependent on them. We've worked really hard to get the computation cost of genome sequencing as low as possible, and we just passed a major milestone last year.

Source: Software Engineer for a well-known non-profit sequencing lab/center.