1) Obviously CPU compatibility
2) Number of power phases, more is better for OC'ing. Also make sure the mobo has a VRM heatsink.
3) Number of PCIe x16 slots. Depends if you will be using SLI/CF. Some have 2 PCIe x16 slots but only one runs at x16, the other runs at x4 or x8 so be careful.
4) SLI/CF compatibility
5) # of RAM slots and compatible RAM speeds.
6) Number of USB headers. USB 3.0 or 2.0 depends on if your case has front panel USB 3.0 you want to use.
7) Form factor, depends on your case or budget. ATX, mATX, ITX. ATX boards usually have more SATA ports, more USB headers, more PCIe slots, more RAM slots etc...
8) Chipset, some chipsets don't support OC'ing, some have more features.
9) Build quality, best way to find out is to read reviews on newegg or another reputable site.
If the $50 one has the same features as a $100 one, meets your needs (OC'ing, gaming, number of PCIe slots etc...) and still maintains solid build quality (high quality power phases, high quality capacitors, and overall build quality) then buy the $50 one. Asus, Gigabyte, ASrock, and *some* MSI make the best quality boards in general. If you have your eye on a few boards, just post them here and for people to give you feedback on it.
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