in a similar idea (Triskter l Identity theft) i explained how i would set the cost (in this case drop the playing cost of the creature)
they had it as a play 6
and steal an ability so i stated that the creature by itself (2l3) would only cost about as much as a horned toad (3l3) so it would cost 2
then the steal ability 3
making the full card worth 5
applying the same
this creature is just barely weaker than the toad, and should also be 2
now if you have to pay
for the steal, the cost shouldn't be added to both the ability and creature, so it only should be 2
with an ability cost of 3
or 5
with no ability cost