The motherboard? Because they are somewhat rare now. Not that particular one, but Z-series boards in general. You can get the 2nd and 3rd-gen CPUs for cheap, but the good motherboards used for overclocking have all been used up. Honestly, I bought one of those exact motherboards on eBay about 2 years ago to go with an i5-2500K. It worked for about 2 weeks. Then it just wouldn't turn on one day. Maybe I was just unlucky, but that isn't the first old motherboard that quit working on me. Motherboards are very intricate and are rather easy to break in one way or another. That is also another reason why they are more expensive the older they get, because you can't buy them anymore and people continue needing them. Motherboards break a lot easier than Intel CPUs.
Simply put, supply and demand. There are lots of K-series Intel CPUs on the market, but not very many Z-series motherboards to go with them. $100+ is crazy. Dont' invest in old hardware that might break on you without a warranty. Get something new if you want to spend money.
*Edit: I just realized you said "My Z77".. you can sell it if you want. I thought you were looking to buy or replace it. I say don't buy them because you don't know where they came from or how badly abused they were.