quoting postGold for Demigod Kills...
I'm not sure if you already get gold for killing enemy demigods... but it must be very little because I never seem to notice it even when watching my gold...

If you don't get a lot of gold for killing an enemy demigod... it doesn't seem like theres much incentive in risking your demigod attacking them if there are plenty of regular spawn units to be killed... of course there also could be a ranking system for killing demigods within games...
I have to disagree with this.  The reward for killing a demigod is that that player will no longer be able to level, build income, or fight against you for a while.  If you make the rewards for getting kills or the punishments for dying too strong, it leads to a slippery slope where a single early game mistake can spell the player's doom for the rest of a game.  ie, player 1 manages to kill player 2 and becomes much stronger due to all the money and exp, when player 2 respawns player 1 is far more powerful than him and easily kills him again.  Player 1 becomes even stronger and all the more easily dispatches player 2 upon his return, and so forth.

Clearly there needs to be something to make taking out enemy demigods worth the effort, but so far I think the respawn times do that well enough.