How much does it take to get from level 5 to level 6?

It varies on what you give, there isn't a hard number. Bundle games only give you 15% of their value, and after the 5th copy of a game, the value you get from each will drop.

https://www.youtube.com/watch?v=l91ISfcuzDw

'bout tree fiddy

http://www.steamgifts.com/discussion/XaCbA/list-of-all-cv-levels-and-their-corresponding-real-cv
As DeltaBladeX says, bundle games only count for 15% towards the next level. Giving away more than 5 copies of a game decreases the value (and this applies over all your giveaways, not just one), and having less than 5 entries removes the value entirely.