Some pros and cons to help decide:
Pros:
-another license or rating looks good on your cv and makes you more employable.
-the FI shows that you hit the books hard and worked hard to learn all aspects of flying, ie: dedication
-you can build time and have others pay the bill.
-you will learn plenty and it will make you a better pilot.
cons:
-the added expense as you mentioned.
-the low instructor wages to start with.
-not too many instructor jobs out there right now.
I instructed for years after not finding work post 9/11. I have no regrets however and gained valuable experience along the way.
I still instruct part-time to keep my FI current.
Good luck in whatever you chose.