The edition determines the cost of each standard CRM license. You cannot mix the various types of licenses. For example, it is not possible to get 5 Professional Edition Licenses and 10 Enterprise Edition Licenses. Either you are in Professional Edition, or you are in Enterprise Edition, and you would have 15 licenses total, given the example above.
In this sense, licensing is pretty straightforward. Note that salesforce.com has an undisclosed pricing matrix, so buying a significant number of licenses may be less expensive than the advertised price. This matrix is updated from time to time, and the pricing cannot be disclosed by customers, so your best option is to contact Sales or your Account Executive, if you already have one.
The prices are not a factor of "cost-saving", but rather, what you intend to do with your CRM. It's hard to tell you which you need, but most organizations go with Enterprise Edition, because it is easily integrated with external systems, has a ton of automation features, and is generally the most functional version for its price point.
I have personally never dealt with licensing, as I'm primarily a developer, but even from a developer standpoint, Professional Edition is great only if you have a small- or medium-sized business that's fairly compact, while Enterprise Edition is where the bulk of the useful features lie. Be sure you take the time to ask the right questions; you can always upgrade later, but downgrading is nigh-impossible for a variety of technical reasons.