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.
Quote above sums it pretty good. To add I'd also say that FI work is loads of fun. The pay might be low at first but will pick up as you gain experience (CPL instructing etc), at least it did for me.
Also FI licence will be an advantage when applying for instructor/training vacancies once you land a job in an airline.
If FI jobs are not available near you, go abroad. There are plenty of countries suffering from lack of instructors since the last reqruiting boom emptied the schools from experienced FI's. You also get to see places and different aviation culture from your own.
Also adding IRI(A) to your FI once you've got the IR experience will broaden your job horizons (doing part time IR instructing is paid quite well).
Good luck,
t. Olabade
ps. Passed my IRI(A) skill test yesterday