Please fill out your profile so we have an idea who we are talking to.
VOIP is basically divided into tow areas:1. VOIP lines, and 2. VOIP telephone systems.
VOIP lines MAY save you money. HOWEVER, if you do use them please please please read about it first. There are still major issues. A major factor that can be overlooked is your internet speed. While there is a minimum speed that the carriers like Vonage will tell you, I would recomend that you add 1/2 that speed again to get decent quality. It is my experience that VOIP lines for small applications are more costly in the end. How often does your internet go down? If it does you WILL loose phone service too.
A VOIP phone system will more than likely only cost more than a digital system of the same size. Of course there will be exceptions, but remember, you really do get what you pay for, especially when it comes to technology like VOIP.
The best way to "go cheaper" would be to lease a system or buy a used system. A used system will fit the bill and be substantially cheaper, you can upgrade it down the road, or replace it with something bigger. Leaseing is another good option, it lets you get something new and makes it affordable. Check out both options.
Steve