A mini suggestion is basically a price suggestion of an item in a price suggestion. You will have to prove the price of the items used in the trade in order to prove the price of the backbiter's billycock.

To do this, look for any sales of the items in the trade to find how much they're worth.

Don't worry though, you don't need to do a mini suggestion of a mini suggestion of a mini suggestion etc etc to keep on proving. If you can't find any usable sales to create the first mini suggestion then just use buyorders like Russell said.

You're fine for this suggestion since the item buy orders plus the keys add up to ~179 keys which rounds to 180. But generally, you should list out each individual item right below the sale compare link, and how you came to each value. Outdated item prices (3+ months old) need "mini-suggestions", and for unpricable items (killstreaks) you take buy order prices or proof of sales. Make sure you don't look at buyers for spells/good killstreakers if the item in the trade wasn't that.

As an example, there is no way you can value a Strange Professional Killstreak Quickiebomb Launcher at 3.18 Keys, when there are more than 10 sellers below that price.