I would like to try the one nearby, so Hongdae is our choice because not all of us are keen on crabs. So just wanna try some is enough for us. I rather take time to go for Hanwoo beef.
1. Famous and cheaper:
馬場洞畜產品市場(마장동먹자골목)
地下鐵5號線馬場站
There's a shop often mentioned on travel blog: 首爾馬場洞龍門家(용문집)
However,from here http://gourmetkc.blogspot.hk/2014/03/blog-post_9.html
someone suggested purchasing beef from hawker counter and go into a nearby restaurant by paying serving fees , you will even cheaper but premium beef.