Calculate Cells based on information is other CELLS

Hello Forum,

I am new to the community and I look forward to hearing your expertise

I need some assistance with the best way to implement the below query which has two parts, I will provide the data I am trying to manipulate as I think that will help me explain:

Part 1

DATA (contained/Found in Sheet 2)

Animal Owner Cost
Dog Theirs 100
Dog Ours 150
Cat Theirs 75
Cat Ours 600
Dog Ours 280

The owner column effectively drives the +ve/-ve value of the cost column
Theirs = positive (+100) Ours = Negative (-150)

What is the best way to calculate a net amount for the Cost based on Animal? or return a net amount for each type of animal?

PART 2
On sheet 1, we will need to pull the net cost amount into the sheet based on the Animal, what is the most accurate way to achieve this?

I am an intermediate Excel user and I have a grasp of EXCEL formulae and some VBA

Thanks