how to i get to madame tussauds using the mtr from causeway bay? is it cheaper to travel by taxi?

How many people do you travel?
If you travel on your own, using MTR is much cheaper than taking taxi directly.
You can go to the Causeway Bay Station to take the Island Line to Central Station and then get out of the station from Exit J2. Then follow the direction signs to walk to the Peak Tram to take the Tram to reach the top of the Peak, on which you can find the Madame Tussauds Museum.

Or a much cheaper method is suggested. You can take the Island Line to Central Station. You can pass the Connaught Place and walk along the Man Yiu Street to the Centural Bus Terminus, where you can take the bus line 15 to reach the top of the peak. The bus fare is around 9.8HKD.