Yes. 20% exactly each time. Just bought a few bloodmobiles and tried it for myself.
XP needed to get to bonus level 1: 483609
6 fifths (how I was taught to add 20% to an existing value) of 483609 is 580330.
Bonus level 2:580330 6 fifths of this is 699396.
Bonus level 3:696396 and so on and so forth.
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