Cheapest would be to do the Fist of Guthix, supposing you can still get experience there. It's free experience, but it is reduced.
Before anything, I'd suggest doing the Witches Potion quest. You should get several levels from this, and it's a very easy one to complete. Check out the
Quest Guide forum for a guide if you feel like you'd need one.
If you're looking for cheap methods that won't get you HP (a very boring method) you could try casting curse while wearing some Melee armor. If you're looking for (I think) cheaper experience that gives you HP experience, buy a bunch of runes for Fire Strike, or whatever is the highest Strike spell you are able to do and camp out at the Lesser Demon at the top of the Wizard tower.
