-Hm. The amount is subjective and no answer will be right. Such little evidence to go off of. However, I believe Kid Buu was weaker than Fat Buu before he split into Good Buu and Evil Buu. That Evil Buu being the fully evil half is essentially equal to unrestrained Kid Buu. Meaning by default, Goku would at full power been surely able to finish Kid Buu off if he could against Fat Buu too. Although, in the manga ssj3 Goku wasn't sure if he would be able to kill Fat Buu, likely do to regeneration and smaller gap.
So. . . I did some calculations when comparing the suggested strength of many character's in accordance to the manga
According to Daizenshuu multiplier
Base: 1
ssj: x50
ssj2: x100 (2x previous)
ssj3: x400 (4x previous)
Janemba = Super Buu more or less: 800
Yet ssj3 Goku(dead) was able to harm Janemba, when he wouldn't have been able to using manga standards. Perhaps Toie's Goku is stronger than manga goku after all? Otherwise, this contradiction make no sense. Inconsistency.
Super Buu(Gotenks absorbed): 1,600
Ultimate Gohan: 1,100
Gotenks(ssj3-rheoretical): 800
Gotenks(ssj2-theoretical): 200
Super Buu(8x): 800
Gotenks (ssj): 100
Goku (ssj3): 105-115
Fat Buu: 100
Kid Buu: 60
Evil Buu: 60
Good Buu: 40
The math points to this extreme difference. And I do not like it either. But it seems to be what the manga points at with daizenshuu multiplier information.
Going by Anime (if Toei' Anime Goku is stronger)
Super Buu(Gotenks absorbed): 1,600
Ultimate Gohan: 1,100
Goku (ssj3): 820-850 (dead)
Gotenks(ssj3-rheoretical): 800
Gotenks(ssj2-theoretical): 200
Super Buu(8x): 800
Goku(ssj3 goku alive): 410-425+ (or 500) - if using around half power from his full power
Gotenks (ssj): 100
Fat Buu: 100
Kid Buu: 60
Evil Buu: 60
Good Buu: 40