GO FOR YOUR DREAMS!
Met a dive instructor last year who gained alot of his training free through dive shops and has been traveling for 4 years - learned lots of languages. Now he is cruising on a live aboard through Papua!
SE Asia has cheap accommodations and inexpensive DM programs - I may do one next summer.
Maybe you can find a pretty cheap flight over there. travelzoo.com sometimes has great deals to the region (Kuala Lumpur, Bangkok, Philippines, Singapore, or somewhere else Air Asia flies) if you are flexible - then link the international flights with cheap air shuttles on airasia.com.
Lots of DM's get partial internships in Thailand - Koh Tao (you should be able to search scuba board or post on the regional section of SB for internship programs in Thailand). Room & Board is usually very cheap. Less than $10 per day. Thailand is fun!
Saw lots of DM's doing cheap training in the north of Malaysia Borneo at
Learn to Dive with Scuba Junkie, a PADI certified operation | Scuba Junkie - (the place is a zoo, DM's are overworked and not sure about the quality of the training, gear is questionable (bring your own reg), but DM course costs about $450, after graduation employment is encouraged and dorms are $3 per night and come with free cheap-cheap-cheap food.
Here is a place in Bali that offers much more professional training:
SSI ITC Bali - PADI Divemaster Internships - SSI Dive Courses Bali - Padang Bai Beach Resort bali - I know rooms can be found for $4-$5 per night and food should be $1.50 per meal.
There are lots of other places cheaper than key largo , just check around!
Good luck!
Once you have picked a location, if start a thread for financial support -- I'll send my $10 to a shop to support your dream! Wish I had started at 18 - life is great, but it would have been much more exciting if I had left the country @ 18!