As someone who was tested recently for certain enzyme mutations that affect drug absorption and effects, it's gotta be pretty cheap for medicare/medicaid to cover it without a prior-authorization. A pain doctor I spoke to says they test for as much as they can get, even told me that one prior patient had been on 5 meds for high blood pressure and after the test showed those 5 were all a type they would not respond to was switched to a new drug and within 2 weeks was at normal levels with just 1 medication. I know that their test can't be too costly, because the lab they use eats the cost that any given insurance doesn't pay (I guess they make up for it by billing other insurances higher).
The biggest problem about total DNA sequencing is that we just don't know what does what. I had a discussion about a misfolding genome disorder that presents differently whether it is inherited from the mother or the father; they know where on the genes it is, they know what protein gets produced incorrectly, but the idea that a set of chemicals knows which parent it's from is a bit staggering when it isn't on the X/Y 46 chromosome. Epigenetics is going to be a huge field, and cheap DNA sequencing along with people willing to share some details about themselves are going to be the fastest way to find patterns and clusters in that mess of data.