implementation cost of VoIP for medium size business

If I understand your question correctly, it sounds as if you are asking about a VOIP Telephone system, not service. If so, I will attempt to answer your question. VOIP systems offer a few advantages over their TDM/Hybrid counterparts. You have ease of expansion without the need for additional cabinets, a single cable infrastructure in most cases to each desk(voice and data are run on one cable) resulting in easier infrastructure maintenance, labeling, and reduced cabling costs, typically very easy moves, adds, and changes, ease of computer telephony integration if required, and a few other minor things. These benefits result in hard dollar and soft dollar savings, and they SHOULD be taken into consideration when evaluation the TCO (total cost of ownership) of your system. Cost can vary widely from manufacturer to manufacturer, so it is difficult to give you a concrete price. The definition of "medium" size business is also very vague. Typically VOIP hardware is still more expensive than traditional telephony up front. The benefit is usually long term, taking into account some of the things listed above. However, if you do not see using any of those applications above, you may consider a traditional telephony solution, as I foresee a large drop in price for VOIP systems in coming years due to increased competition and improved manufacturing volumes. If I can assist you, feel free to drop me a line. Thanks.