10 hours billable --- 2 out of 5 work days --- can easily wind up being 20 hours of wall clock time, factoring in ramp-up, off-the-clock support, documentation, non-billable research time, and then the overhead of dropping in and out of your product.

Of our original 3-person founding team, excluding all the hires since then, we've found that none of us can keep up with the FT devs, even when we work out one-week scheduling bubbles. This is obviously a classic "flow" problem.

I'm from dev originally, not consulting, and I was surprised by how expensive (timewise) services work is.

Services is expensive, but you should be billing for some of those things like off-the-clock support, documentation, research, etc.

Depends on the customer, but we never bill for rustproofing. Get used to bending over backwards for people who pay you now; good survival skill. Truism: it's the perception of customer service that people appreciate, not the execution of it.