Who says you'll never learn anything hanging out here?
Used in this sense, the phrase "to boot" comes from middle English, a word meaning remedy. In this case, the remedy for an unequal trade- and what is asked for- "to boot"- is to make the trade even.
"If you'll swap me your shotgun for my rifle, I'll throw in 5 boxes of ammo, to boot."
"If you want to trade your .357 for my .45, I'll need about $100 to boot."