The basic network requires, a router to handle the broadband connection, a switch to multiply the amount of end points and IP phones. IP phones can come as physical devices (Hard Phones) or as an application that works on your PC, Mac, Android or Apple mobile phones.(Soft Phones)
As with all these things you can spend a little or a lot. The good thing about IP Phones is that even at entry level the majority of functionality is available. Most companies need a phone to do the thing it was intended for, 'making phone calls'. The increases in handset cost usually come down to physical features being added on such as bigger displays, colour displays, more buttons, more buttons with flashing lights!
Entry Level phones start around £40 and you can go up from there