First, if you are reading this...I'm sorry, I wish I was a better writer. :D I try to avoid using always and never, opting for a little less definitive "almost always" or "almost never". I've tried to use them correctly on this page.
While testing a few things I've noticed that encounters that give a reward seem to have a variety of reward "types". First there are the gold and/or mana rewards. These range from 10 gold or mana up to 200 or so of either. Sometimes an encounter will give both, but usually it is one or the other. These encounters will normally have only a few defenders, if any, but I've noticed that the reward is always the same in a given game. In other words, if you defeat some of the defenders but not all of them until a later encounter the reward hasn't changed.
The second type of reward that I've noticed is the items. Encounters that give items seem to always give a different item unless you reload and do almost exactly the same things, and even then the item can change. I think this is the thing that changes most often and has lead to the conclusion that the reward value of an encounter has changed. I believe that the reward values are determined at the start of the game, much like the locations of the nodes and the distribution of the races.
A little background regarding my conclusions, after playing about 20 games I noticed that I always seemed to get the same 30 items plus a few that were different. Of course, the different items were usually higher quality items and they replaced some of the items I got in a previous game. To jump forward a little, I was always disappointed in the number of jewelry type items that I was getting (this was before I started using alchemy, runemaster, etc) and so I edited some of the 30 items that I seemed to always get and turned them into jewelry. After that, I seemed to get enough jewelry to outfit my heroes instead of having to make those items during the game.
Back to the reward part, I had noticed that reloading and retrying an encounter would give me a different item most of the time. That was too tedious to do very often, especially if I wanted a certain type of item. Anyway the point is that the encounter that gave me an item almost always gave me an item. The mana value of the item seemed to vary a LOT, but it was still an item. I think this is specifically the area that has generated the most belief that the reward value of an encounter changed. My point is that it didn't really change, it was an item and it is still an item and that the random pick of the reward has changed, not the reward value.
On the the third type of reward and that is encounters that get you a prisoner. This is where I really started to think that the reward value didn't change, just the reward. As long as I still had an opening when attempting an encounter that rewarded a prisoner, I always got a prisoner. The prisoner may have changed, and in my mind I may have gotten a "better" reward or a "lesser" reward, but it was still a prisoner. When I attempted the encounter and DIDN'T have an opening for a hero, I still received any other reward that I got previously...I just didn't get a prisoner.
And finally the fourth type of reward and that is a spellbook/retort or two even. Based on my testing for this, it is just a matter of getting this reward. This is the only type of encounter when I have seen a reward change "types". While I couldn't test EVERY encounter in a single game, I believe that I have checked enough to conclude that as long as you qualify to receive a spellbook or retort you will receive one from this encounter with the occasional random change to an item instead. In other words, if you receive a spellbook or retort and an item, and then reload and attempt the encounter later (or again) it is possible that you won't receive a spellbook or retort and instead will receive two items. One item is to replace the spellbook or retort and one is the same as with the first attempt. An encounter that gives two spellbooks or retorts AND an item at first can give as many as three items, but may only give two items if a spellbook or retort isn't an option.
In an attempt to summarize: I've concluded that there are 4 types of rewards. I've concluded that the reward value is established at the start of a game and doesn't change for an encounter. I've concluded that the reward types are: 1) gold or mana, 2) items, 3) prisoners, and 4) spellbooks or retorts. I've concluded that the first three types do not change types of rewards, and that the fourth (spellbooks/retorts) can randomly change to an item instead of a S/R.
These are my current thoughts regarding rewards and I'd like to hear yours if you would like to post a comment or reply to a previous comment.