Some faction rulers have the greedy trait. You can use this to make them your puppet/tributary state. Here I was Westphalia making Charlemagne my tributary state.
Turn 1, I bought peace for 2800. I've seen the amount vary from the 2-7000 range. There seems to be several variables, I haven't exactly figured out what effects the amount.
http://i.imgur.com/iQ04w7J.jpg?1
You might be able to get a non aggression pact as part of the above deal or right afterwards.
http://i.imgur.com/cgVe4Sj.jpg?1
This next part depends on how quickly Charlemagne absorbs Carlomans territory(turn varies). The price will go up significantly, double or more, If your unable to make Charlemagne your tributary state before the merge. In this case I was able to do it before the merge. The merge occurring about 8 turns later.
http://i.imgur.com/gPowv2k.jpg?1
You'll want to keep Charlemagne involved in at least 2 to 3 wars if possible. Save up some money, declare war on a faction then pay Charlemagne to join the war, try to declare on small factions first and slowly work your way up to the larger factions. Helping Charlemagne with battle support along with the joint wars will help to keep him loyal.
http://i.imgur.com/OZ1ldDW.jpg?1
http://i.imgur.com/hDtlY4W.jpg?1
Video of the first 20 turns
https://www.youtube.com/watch?v=oMLXBRGdXhw