Depends if you buy F2P or P2P leeching. Also, if you buy dungeoneering leeching, you are guaranteed xp. However, with spins it is chance. But you could always win a spirit shield from a spin.
At your level, doing like the last 5 floors large. It would be around 30k exp per floor. So I'd say around 400-500k, more or less, and you'd level up a few times as well so you'd be able to do higher floors. All in all, I'd say about 500k exp.
Dung leeching for sure. I saw someone get only 100k exp in their desired skill after buying 75 spins whereas the leeching would get you around 450k exp I'd say + tokens