what is the difference between a cheap and a expensive motherboard?

Both those replies are wrong. MoBos can vary greatly, actually.

Things to look for:
-Clock speeds for RAM
-Bus speeds for ports or connectors
-Voltage needed to operate (sometimes other small things that need to get plugged in, particularly power button but that's also the case)
-Thermal Coating. My last MoBo had a ceramic coating which inflated the price by like 30$
-Socket for CPU (You'll find AMD sockets are actually generally cheaper. Also older sockets which arent usually supported will have higher prices)
-Size (Mini vs full size)
-Number of ports (having 6 USB3 ports and 2 USB C versus 2 USB3 and 4 USB2, or how many PCI-E are available, also has to do with size)
-Any integrated parts, like not needing a PCI-E network adapter