I made two nearly identical mixings in Dash core and Dash electrum, and I've noticed that I only paid 0.0001 Dash with electrum while I paid 0.0003 with core.
Is the collateral amount determined by the wallet or is it random?
The amount paid depends not just on the collateral, but on denominations you create and mix. In Electrum we try to minimize the fee you pay while mixing.