You can only have one device normally connected to a dedicated modem at any one time, hence the IP conflict.
To have multiple device connected by cable and wireless, you need to have a router connected to the modem, which will then issue IP's on your own private network and handle the routing out to the outside world on the IP assigned by your internet provider.
Now the big question how much?
All depends on what you want, a cheap router can be from £25, but may limit network performance or be hampered by your building construction due to low power wifi radios, while an expensive once, anywhere up to £150+, will do everything, will have better network performance, more powerful wifi radios, more memory, faster processors and provide both 2.4ghz and 5ghz frequencies and more.