Interesting problem, but won't work.
For example, I can pay for xp costs with other sources. This means that the xp cost of say... casting simulacrum is a cost, not a xp increase. In order for your concept to have any traction, you need to have a negative XP award.
For example, killing a monster grants you 200 xp in situation X.
However, in some games, killing a NPC is a XP penalty of say -200 xp. (why did you kill all the kobold children?)
In THAT case, and that case alone would your theoretical -20% penalty net you a whopping 40 xp.
However, the only examples I can find of this are in modules. Therefore there is no hard core rule mechanic where you can consistently exploit this.
Spending XP is not the same as EARNING XP. I am not EARNING negative XP when I brew a potion. I am not EARNING negative xp when I cast a spell with an XP cost. I am spending XP that exists, not being given Anti-XP.
Although I am now intrigued with the very concept of Anti-XP. I think it might make for a very strange concept in a game. Sort of thing a villain embraces, or the existence of Nightmare world creatures (Beings that exist 3 dimensionally in dimensions 3,4, and 5, as opposed to dimensions 1,2,3) that work on the force of anti-XP.
Fascinating. I must explore it sometime.