The cost of a 100-foot Ethernet cable is mostly dependent on the category. You can get a 100-foot Cat 5 Ethernet cable for $20-$30, but a Cat 8 Ethernet cable of the same length may cost as much ...

Amazon, however, is offering a 50-foot flat CAT 6 ethernet cable for just $6.90 right now. That makes it pretty cheap to reach the closest router from out-of-the-way corners in your home.