Which according to you raises your trading cost the most?

Choosing the broker's low spread I think is a matter in trading, it will give more benefit when trading at a low cost like as getting a zero spread or even a negative spread, it is possible to trade get a negative spread using Ticktrader terminal in FXOpen

Appropriate money management policy helps a trader bear profit on average. But those who want to earn big amount of profit and are never satisfied with average profit are losers ultimately.

I think those cover the majority but there are some parasitic brokers that also charge for withdrawals and deposits.

For me it's the wide spreads.

I think the most costly would come down to either spreads or commission. I'd also consider other factors...some brokers may charge for account miniatous, inactivity, and withdrawals.

I think it completely depends on the methodology a trader uses for trading and the duration of trades.