I have some items that gives me +X experience points for each kill, or +X%, but how does this compare to the XP obtained by killing those monsters? I can't see how many points I get when I kill them.
In general, from about Normal difficulty Act 2 onwards, a +10% or higher bonus (from putting a Ruby into a socketed helmet) is going to beat the pants off of any +XP bonuses you can find.
The +XP bonus enchant tends to max out around +15 XP, which would mean it would beat a +10% XP enchant if monsters were worth less than 150 XP per kill. About midway through Act 2, I sampled some of the monsters XP values as I was killing them, by monitoring my current XP to level before and after encounters. Routinely, the average per-kill was more than 200 XP. As you progress through the game, the amount of XP per-kill will increase, and easily dwarf the small bonuses you can get from +XP gear.
This doesn't mean that you should not use +XP enchants on other equipment - but especially later in the game, it becomes less and less worth the investment. Almost always, if you have a choice between a socketed helm with a good quality ruby, or a non-socketed helm with a +XP enchant, take the helm with the ruby.
Bear in mind that the XP you gain from monsters also depends on your level versus the level of the monsters, and information about monster levels is not readily available in-game. Thus, the only real way you can say how much the monsters you are fighting are contributing to your XP is to hover over your XP bar before and after killing them, and then do the math based on these values.
Since patch 2.1.2 the "Monster Kills Grant +X experience" secondary affix is multiplicative with your Bonus Experience %.
Sadly, I don't have any sources, but I've done some extensive testing and I found the following formula for XP per kill:
XP per kill = (
Basically, the system works as follows: every monster has a certain base XP. For instance, a Risen on level 1 give you 30 XP per kill, but only when you are level 1 as well. Once you start gaining levels, you lose 10% XP for each level that you're higher than the monster you just killed. When you enter Old Tristram Road at level 1, then all monsters will spawn at level 1. Should you reach level 2, then the monsters will not automatically level with you and you'll start receiving 10% less XP per kill (27 at level 2, 24 at level 3, etc.). Only monsters that spawn from this moment on will be at your level, such as Risen crawling out of the ground or graves, or when you enter a new area.
You don't get more XP when the monster level is higher than your level. I started a level 2 co-op game, had my level 1 character join, made the level 2 character spawn some monsters, made the level 2 character leave and let the level 1 character kill the level 2 monster: it gave the regular, level 2 monster XP.
Anyway, the amount of XP that the white version of a monster gives without any bonus XP modifiers on Normal mode is what we'll call
When you open your inventory screen, and open the details, you'll see two items under the heading Adventure which contribute to how much XP you're earning: Bonus Experience and Bonus Experience per kill. The first item is split in two separate points, the descriptions are from the game itself:
The second point is not entirely true, as activating a Shrine of Enlightenment will increase this value by 0.25.
The second item is
The last is
With the Flawless Royal endgame gems, you can have +41% XP on your helmet, or 82% with a well-rolled Leoric's Crown. Note, that the percentage you can get from gear (Ruby in Helm, Level 70 Hellfire Ring, etc.) is additive with the percentages you get from difficulty.
With the recently added Gem of Ease, you get at least 500 bonus XP per kill and per rank you'll get an another extra 50 XP on top of that. This is additive with the +XP from kill secondary affixes.
Grabbing the above formula, a level 50 Gem of Ease, 3380 bonus XP per kill, a Pool of Reflection and Torment VI, a Risen will yield 597762 XP, as opposed to 369750 XP on just Torment VI alone without any of the modifiers and 462187 with a Pool of Reflection. So in here, the +XP per kill grants you 135575 extra XP, but at the cost of DPS from the Emerald in your weapon and probably some more useful secondary affixes.
Excluding all the mechanics of how much bonuses do increase your XP by (which you can readily see), here is what developers mentioned about approximate worth of monsters (in 2.1.2 patch notes):