That seems pretty UP.
3 + 2 (already worth 6 ) + 1 turn for a 6|3? Bad.
3 + 1:time + 1 turn for 8|3 upped? That's still pretty bad.
The problem is come into play invulnerable, creatures that are invulnerable, tend to be more expensive, we see the aether with its immaterial creatures, these calculations of their balance is good, but notice the effects of abilities in the game is also just as good..
an example is the lycanthrope, it is a bit cheaper, but often fail to use their ability, they can be easily destroyed.
Except creatures with the Burrow ability do not seem to have a large cost increase. (Unlike creatures that start protected)
The existing version of Graboid (cost, equivalence, effect, value)
3
+ 1 card, 3
+ 1 card, 2|3 Burrowed, 2
+ 1 card
3
+ 1 card + 1
+ 1 turn, 6
+ 1 card, 8|3 Burrow, 7
+ 1 card
By these estimates: Only the ability is OP and only by 1 quanta.
weakened majofa's version (cost, equivalence, effect, value)
3
+ 1 card, 3
+ 1 card, 2|3 Burrowed, 2
+ 1 card
3
+ 1 card + 2
+ 1 turn, 7
+ 1 card, 6|3 Burrow, 5
+ 1 card
By these estimates: Only the ability is unbalanced. This time it is UP by 2 quanta.
full majofa's version (cost, equivalence, effect, value)
3
+ 1 card, 3
+ 1 card, 2|3 Burrowed, 2
+ 1 card
3
+ 1 card + 2
+ 1 turn, 7
+ 1 card, 6|3 Better Burrow, 5
+ 1 card
3
+ 1 card + 2
+ 1
+ 1 turn*, 8
+ 1 card, 3|3 Momentum Burrowed, 6
+ 1 card
By these estimates: Only the ability is unbalanced. This time it is UP by 2 quanta even if the 2nd ability is used.
*Turns count advantage offset. In this case the damage offset. However the brief period unburrowed caught the damage up so it is not as great a loss.
majofa's Graboid + existing Shrieker + majofa's Burrow looks like ideal balance (cost, equivalence, effect, value)
3
+ 1 card, 3
+ 1 card, 2|3 Burrowed, 2
+ 1 card
3
+ 1 card + 2
+ 1 turn, 7
+ 1 card, 8|3 Better Burrow, 7
+ 1 card
3
+ 1 card + 2
+ 1
+ 1 turn*, 8
+ 1 card, 4|3 Momentum Burrowed, 8
+ 1 card
Conclusion: majofa's Graboid + existing Shrieker + majofa's Burrow looks like ideal balance