I was wondering if someone had any idea of what would the CR value of the ability be. In other words what would be the CR adjustment to apply a kind of golem template to a random creature.
The effigy creature template in Complete Mage has a +1 CR, but no magic immunity.
I seem to recall there being a Half-Golem template in 3.X MM2, though I don't remember the specifics of it. Perhaps you could start your search there?
You might compare the golem's stats (hp, average damage, AC, etc) to the monster building tables to establish a CR for a non-immune golem, and then take the difference?
For example,the stone golem--I've listed stats on the left, corresponding CR value on the right.
hp: 107 8
AC: 26 11-12
Saves: 4/3/4 1
Attack: +22 *2 13
Damage: 20 5
That all averages out to somewhere in the 7-8 range (more or less ignoring the saves part because it didn't need them before). It's listed CR is 11, so then magic immunity is worth +3 or +4 CR.
It's fairly rough, but it's somewhere to start.
At low levels, I would say that would be worth a feat, or something along those lines. However, it gets better and better as the CR increases, so for a CR 20 creature, it might be worth a +3 o +4 to CR. It also depends fairly heavily on what type of creature it is. For a vermin, animal, un-intelligent undead, construct, it isn't that big a deal. For a demon, devil, caster, or dragon, it could be a huge advantage.
CR
1-5 - No adjustment
6-10 - +1
11-15 - +2
16-20 - +3
I might treat dragons, outsiders, and humanoids with PC levels as 1 higher CR, before applying the adjustment.