I remember pricing out a diesel generator from a local company, and it was going to cost $10K with another $4-5K for installation. I bought one myself for about $7500, and did my own install for about another $1200 in materials, another couple hundred for equipment rental, and many hours of labor. After going through that experience, I could easily appreciate the $4-5K install cost, especially for a professional company that would already have their own equipment, use licensed workers, and have stricter procedures to follow with codes, inspections, etc. The combination of electrical and mechanical work, plus site prep, trenching etc, make these installs pretty comprehensive. It would have been a little simpler if I put it next to the house, but my generator is about 20-25' away in a small outbuilding.