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