Start by carefully diagramming out the devices in use and the ports/connections and the cables you wish to use.
Sketch out/diagram ports, plugs (each cable end) between computers (2 or 4 if I understand correctly) and the keyboard, mouse, and monitor.
Pay careful attention to genders and the possible need for adapters.
For example:
http://www.highseclabs.com/products/?pid=78
Note the "System Diagram".
Your diagram does not need to be professional - just enough to clearly show the required connections.,
You can easily find other example diagrams.
To answer the
second question first: the price is likely to depend on the details starting with a decision on how many CPUs (computers) are being managed via the KVM.
Followed by what physical connections are needed: USB, HDMI, DP, etc..
First question: DP is the newer video technology so go with DP if your monitors have DP. May cost more though. HDMI may work just as well and the KVM could be less expensive.
And do not forget to consider the extra costs (if any) for cables and adapters may be required.
The diagram will help you narrow down product (KVM make, model, configuration) options and that will, in turn help determine the potential costs involved.
Then it becomes a matter of product reviews and related research.