Currently there is no dedicated medical expense tracking system within Shelterluv for in custody animals. You can however create a custom memo type(s) to track this information which can be pulled into a Custom Entities Animals Report. Each memo type has its own column in the reports table, so you could potentially add separate types for Expense Description, Expense Amount, etc. Keep in mind that only the most recent memo of each type shows in reporting so you'll want to utilize and update one of each memo type per animal as you add additional expenses.
Alternatively, if you are tracking expenses for each vaccine, treatment, surgery, etc. you can pull custom events reports on these completed medical tasks. Custom Events Reports would allow you to see these specific vaccines, treatments, surgeries, etc. which you can then calculate costs for, once the data is exported to excel (see How do I run custom reports?).