How much would it cost to get 75 construction from 27? And what's the fastest/most cost efficient methods?

27-75 is alot.

 

Just make oak stuff.

 

Say, how many larders you need to make for 75, times that answer by 8 and thats the amount of oak planks you need, then times that by oak plank costs.

 

Do

 

NOT

 

Run your own planks.

 

 

 

Calcs: http://www.tip.it/runescape/?page=skill_calculators.htm

It should cost about 7.5m - 4.3ish if you use Sacred Clay hammers.

 

You'll be buying about 20,000 oak logs, un-noting them using your butler and using him again to plank them. Make oak larders, then dungeon doors at 74(?) construction.

 

 

 

Just so you're aware, you can make gilded altars at level 69 construction.