Romania. Even its capital and the largest city - Bucharest - is still pretty cheap, sometimes even free like in this example on Google Street View https://maps.app.goo.gl/r6TFFtHbj2SELTqY9 If you're willing to take the risk which is pretty low, you can even park it on the sidewalk like here https://maps.app.goo.gl/y6DNVBdR2KvJsA917

But times are changing. Lanes and sidewalks, sometimes even green spaces, are being converted to parking spaces, so there's less spaces for freeloading. They're also becoming more and more expensive. The residential ones have also been hard to get and it will probably become even harder to get as more drivers will need them as the risk of getting a fine increases.