Warren wrote:
Oak larders, no question. Always use a butler to retrieve planks from your bank and possibly go to the sawmill to buy planks. Otherwise, to create planks, do the Enlightened Journey quest, unlock the varrock route, and get a pile of rings of dueling. Tele to Castle Wars, bank, take the balloon to Varrock, plank, repeat.
I know you don't want to go past 60, but for future reference: Switch to oak dungeon doors at level 74.
That's by far the best way to lvl con, right to 99.
The CHEAPEST however is limestone fireplaces. Truly i wouldn't bother with this, whilst it's cheap it takes forever, and time = money. You can easily but oak planks at mid price, stick to the previous posters advice.
Bear in midn the Ge will only let you buy 10k at a time.