more a rhetorical question than anything else-
Why are mindreading costs in FoV so damned high?
They are not. It just seems that way. Yes, you can quickly ramp up a debt of like 300k+ but given the amounts of XP you get in the game it's really neglectible.
Also, the game adjusts the xp you get from monsters by your own level. What does that mean? If you level slower due to mindreading more often, you get *more* xp than you would have gotten if you have saved all those mindreading-xp.
You still pay a bit more without mindreading but in the end, I doubt the difference is that actually big (if it makes up for two levels at the very end, I'd be highly surprised).
Also, putting skillpoints into mindreading (and wisdom) does not make any sense at all. As every skillpoint you get equals (roughly) one level, each point in mindreading and wisdom has to bring you a level in savings or it's a point you should spend elsewhere.
Given the xp scales to your level increasing wis and mindreading will never pay off.
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Now that I wrote that I feel curious how it actually works out.
I'm not sure if the numbers are correct, but I have the impression, you get 10% more xp for every level that is below the monster you kill and 10% less for every level you are above it. At least the rough region seems to be correct.
So if a level 20 monster is "worth" 3000 xp and you are level 20, you get 3000, if you are level 19 you get 3300 and if you are level 21 you only get 2700 xp for killing it.
Let's build some sample players: Player A has put skillpoints in Wisdom to get 5% more xp (like 3 or so?) and does not mindread at all as he wants to save those xp.
Player B has two skillpoints in Mindread to save like 20% of those xp and also has some points in wisdom (like 3) to also get 5% more xp. Player B mindreads people worth 50.000 (and due to his 20% savings he only pays 40k for that).
Player C has no points in wisdom, none in mindread, but also mindreads everyone he meets.
Both start with the same level (let's say... 18) without any XP debt (like... at the beginning of FoV) and player A never mindreads but Player B collects a debt of like 50.000 due to enjoying the nice things Larian put into the game (and gets extra stuff from mindread-stuff).
Player D does nothing. No mindread at all, no skillpoints in wisdom.
Let's assume you get a level every 50.000 and that both kill a dungeon of 50 monsters (those mentioned above worth 3k each).
The players all get 3600 per monster as long they are level 18, 3300 per monster if they are level 19, 3000 per monster if they are level 20, and 2700 if they are level 21.
So, you can get 150k base-xp from those 50 monsters. Let's see what actually happens.
In the following calculations, I will round fractions of monsters to full monsters.

I do not know how the xp for wisdom works, meaning if you get 5% of the BASE xp cost extra of 5% of the total xp (scaled by level); I will assume it's scaled by level.
Player A (0 debt, +5% xp, has spent 3 skillpoints aka 3 levels):
Level 18: He gets 3780 xp per monster and is level 19 after killing 14 enemies.
Level 19: He gets 3465 xp per monster and is level 20 after killing 15 more enemies.
Level 20: He gets 3150 xp per monster and is level 21 after killing 16 more enemies.
Level 21: He has 5 monsters left and will get 14k xp for them.
Player B (40000 debt, +5% xp, -20% on mindread, he has spent 5 skillpoints aka 5 levels):
Level 18: 3780 per monster and is level 19 after 27 killing enemies.
Level 19: 3465 per monster and is level 20 after killing 15 more enemies.
Level 20: He has 8 monsters left and will get 25200 xp from them (3150 each); this puts him halfway into level 21; he has 40k less than player A (his debt).
Player C (50000 debt, no bonus xp, he spent 0 skillpoints aka levels):
Level 18: 3600 per monster and he will be level 19 after 28 monsters.
Level 19: 3300 per monster and he will be level 20 after 16 more monsters.
Level 20: Has 6 monsters left and will get 18000 for them.
He has 46k less than Player A and 6k less than player B.
Player D(no debt, no bonus xp, no skillpoints aka levels spent on getting more xp):
Level 18: 3600 per monsters, he will be level 19 after 14 monsters.
Level 19: 3300 per monster, he will be level 20 after 16 more monsters.
Level 20: 3000 per monster, he will be level 21 after 17 more monsters.
Level 21: 2700 per monsters and 3 remaining monsters: 8100 xp.
He has 6k less than player A, 46k more than player B, and 52k more than player C.
When comparing the players I conclude (for myself):
Player A passes up all extra rewards (including skill- and statpoints!) and misses the story to gain a bit more xp. His 3 skillpoints he put in wisdom make his character significantly weaker than one that did not spent those points (Player D). To make that pay, he has to be at least 4 levels higher than Player D *at the end of the game* but paid for that with a character that is weaker over 90+% of the game's playtime.
Conclusion: Skillpoints in wisdom don't really pay.
Player B wants to mindread and also wants xp, so he spends 5 skillpoints on mindread and wisdom. If compared to player C, it turns out that player C will be one level behind - but has not spent the skillpoints of five(!) levels to have that slight advantage. Player C got all the advantages (rewards and nice story) as player B did.
Player C is a few thousand xp behind Player B and it *might* total up to a full level at the end of the game (in addition to the level or two you might get from the wisdom), but Player B must be at least six Levels ahead of Player C in the end to make this setup be worth the points.
Conclusion: In addition to points in wisdom being a waste that does not pay, skillpoints in mindread do not pay as well.
Player A and D are roughly the same level, but A is like three behind due to his skillpoint-usuage.
And the XP to pay the debt in the AddOn come fast enough anyway, also.. does it matter if you are level... 46 or 44 at the end of the game? Not really, I think.