Let's try to jump-start this discussion...
It appears more people have coins in their accounts as opposed to unredeemed club nintendo codes just laying around. And a lot of people just want to trade those for other codes instead of selling them.
I've purchased a 600 coin downloadable game outright for $12, though I believe that was on the low end. (You may end up in a bidding war and pay around $15) So you would think $5 could get you about 300 club nintendo coins, but admittedly, that's 4 Wii U titles and a 3ds/wii/ds title to make up the remainder. People are going to value that higher than five dollars.
Alamo sold me a 60 coin code for $3. I needed just 180 coins and ended up paying around $11 between here and ebay to get those.
If you do look on ebay you will see that $5 gets you about a 1 Wii U CN code (70 coins). But don't fall for the Wii Mini codes (100 coins). They're cheap but they are fraudulent.
If you're lucky, I'd say you could get two 3DS codes (100 coins) for $5