According to the latest survey by real estate agent Savills, London is twice as expensive as Sydney and is four times costlier than Brazil’s capital Rio de Janeyro.

Can I say ... [i]four times as cost as[/i] ...? Are they interchangeable?


That would have to be
… four times as costly as …


(1) Their house is about [i]three times as big as[/i] ours.

  Similarly, can I say as follows:

 (2) Their house is about [i]three times bigger than[/i] ours.

   Thanks a lot.

Relying on what follows, yes, you can:

"I’m pretty sure I usually say “three times as big as”, but I could also happily say “three times bigger than”.

I’d never use the comparative with “twice”, though… .

You’re safest with “as big”, I think."

I concur.