It's not supposed to be balanced though. Rumor quests shouldn't be something you always play everytime. The decision to play them is nearly always in the overlord's hands (except when the rare card to force him to play one shows up). That means it needs to be a risk for the overlord to play them, or else they'll ALWAYS play it and then that mechanic is pointless. But it also needs to be enough of a potential reward for the overlord for him to want to play it sometimes.
Rewarding the overlord with an extra experience point if he wins means that it's worth it to play for the overlord if he believes he will win (especially if it's his quest choice), but if he's not sure of it, it's a gamble. In this way, it's no more a big deal for the overlord to play it than say, A Fat Goblin or Castle Daerion where the prize is extra gold for the heroes or extra xp for the overlord. Also, the extra XP might make the players less likely to spend time grabbing all the treasure on these rumor quests and instead try to guarantee a win, meaning even when you lose as overlord, you don't lose as much relatively.
You say that it should be a risk for the OL to play it, yet a risk with little return is never going to be worth it. Then we get stuck back at the point where an OL is still not playing OL cards because the injection of gold into the game makes the heroes uber powerful - giving the heroes a 37% more gold is not a risk, its virtually handing them the game.
Sorry but i do not feel that just adding a 1xp to the quest rewards for the OL alone fixes the problem, it might be part of a solution but i do not believe its the entire solution.
I am not trying to make things perfectly balanced, however the current rules are so skewed to the heroes that the OL would be insane to ever play a rumor quest if he wants to win a campaign.
I disagree. I have had times where, knowing the party's weaknesses and layout, have chosen to play rumor cards when it was my quest pick to force us to do it immediately. The party got destroyed hard, I got my relic, they got almost no loot.
This was before we implemented that houserule too. Gold is helpful for the heroes, no doubt, but it's not nearly as big a deal as some people are making it out to be. There are already quests which have the rewards being 25 gold per hero vs. 1 xp for the overlord. Yeah they might get some loot (honestly though, it's not as if they're guaranteed to get all the loot. And if you asked me if I'd take Her Majesty's Malice and an experience point in exchange for letting the party have 3 or 4 search tokens, I'll almost always say yes to that.
The main issue is that the overlord isn't guaranteed a victory, so thus the risk.