Depending upon the phone technology, expenses will vary, with KSU-Less systems being the least expensive due to the lack of a central control box, and no added costs for maintenance and installation. KSU Systems are more expensive due to these added features, typically costing around $250 (including installation). PBX is the most expensive, starting at around $1,000 per employee. VoIP systems are known to be inexpensive, and our favorites average $15 to $20 a month per user.