There are issues with going to the least expensive medical school that will come with trade-offs that might not be interesting or desirable. Some of the least expensive medical schools have issues with accreditation or steer doctors towards a specific specialty.
There are however ways to get medical degrees for nothing at all and one of them is doing it through one of the armed forces. This route comes with a requirement to serve in the military for a certain number of years after your medical training has been completed. Getting a medical degree paid for by the military is also very very competitive and your grades have to be higher than pretty much every other person on the planet.
You are fortunate in a since living in Puerto Rico however as there are some medical schools there that are accredited and less expensive than many others. Check into the requirements and you may find that they are a perfect fit.
best of luck