Why has my value gone down ?

Something must have had its price reduced.

¿Bundle games? ¿Price drops? Is either one of those or the other

It happened to me too

So if i giveaway a $30 game today and the giveaway ends today in 6 months when its $5 my value goes down to $5 ?

Most you've given away are bundle or some sort of exploit games and I believe nuclear dawn only became one of that category recently.

And price drops yeah.

On top of that; after a year the value of a giveaway stops being counted at all, I vaguely remember.

Nuclear Dawn was recently re-added to the bundle list, that might be part of it.