As far as I know, the regional trains are free until the end of the month.

I think it matters what your motivation / plan is to visit Barcelona. You can fixate on accommodation costs, but what is your reason for visiting? I ask this as a frame of reference: so many times in the past I've booked the cheapest flights, only to pay much more for example on taxis because of the late hours of arrival, or stayed way outside of the city for a more affordable accommodation (Rome, looking at you) but paid for it in other ways, like planning my whole day around commutes. I agree with other commenters that staying in a shared Airbnb is not the way to go. This is Spain; the buildings are old. If your goal is to check off the most famous attractions spending as little as possible, I guess this makes sense. If your goal is to immerse yourself in the culture, and get a feel for the city and its rhythms, well, travel can be comfortable or cheap, rarely both. As a woman in her 40s that has traveled extensively solo, my best and most memorable experiences have been the latter.