You always pay half the fee. This, therefore, has a standard margin of 1%. Depending on your VIP level, you can earn a higher discount.
VIP level 9 gives 1.75% discount. This leaves you with a fee of 0.25%. Because you pay half of this fee, you pay 0.125% for exchanging the coins directly.
It is, therefore, cheaper to change a coin directly for another coin than to sell it and then buy another coin.