There are plenty of AirBnB places in fairly touristed places in the developing world for 15-20€/day.
Room or a whole house?
Using Airbnb, I rented an entire 2 bedroom apartment in downtown Belgrade, Serbia for about 13 days. With fees, the price was $380-ish USD, just under $30/night. It was a really good deal!
In many countries, in that price range you can find everything. For example, last year on a cycling trip I ended up in an Albanian city on the one weekend a year when all accommodation is fully booked due to a festival. Lucky there was one remaining AirBnB host, and I rented his five-room house for 25€, fees included. He lived elsewhere, I had the whole place to myself (which was total overkill). Most of the other hosts, though unfortunately already booked, were offering rooms or entire spare flats for half that.
Hotel rooms, too
I rented (entire) Airbnb places in Berlin for one year because I was looking for a flat.
I used this to scout each part of the city to decide where to look for rentals. I was always traveling during weekends, so I mostly rented during the week.
There are top notch AirBnB apartments in Berlin that you can get for as little as 20€ a day.
A rental pretty much anywhere in Poland will be around that. I've paid as little as $20/day for a nice flat.