Its hard to give a price break down of every minirank, I can try and give you a general idea though. Btw Bus's PC's are a little low, but I think he's just good at haggling and I don't doubt he found good deals.
Most holiday or common miniranks are worth under $10. Examples I <3 N4n0 (a bunch we're given out) or saint Patty's day minirank.
The average minirank goes for $15-25. It really depends on how the seller likes the rank, what they paid, how desperate they are to get rid of it, if you're friends, etc. Some examples in this category would be like doge or mushy muncher.
Then on the upper end you can be looking at $30-50. Rare or nice looking ones generally go in this price range. Examples would be Legion, Weed, Ninja, The dark side, etc.
You should never really pay more than that, unless you're seriously collecting and only 1-2 people have the one you really want.