How much is killing heroes worth?
The experience you get from killing creeps/towers/forts/heroes are granulated according to team level difference. As stated in this answer :
The experience gained from either of these actions are not constant and is granulated for the purpose of preventing snowballing effect, therefore its not possible to tell an exact number on these XP sources. It depends on your level and enemy team's level.
As for the team fights in the beginning, you'll notice more experienced players in the pro scene will avoid that initial 5v5 team fight as it has no real purpose (not related to any objective or lane pushing), and will go to lanes or group up for an early gank, and then go back to lanes.
Edit: This reddit post has some detailed answer about the numbers.
I strongly recommend anyone curious about the topic read the reddit link, everything is in detail there. Here are some highlights:
Buildings have set values for XP.
- First wall turrets 400xp
- Forts 800xp
- Second wall turrets 650xp
- Keeps 1300xp
Hero kills give a base level that is modified by being behind or in the lead.
I believe the base number is 300xp, being two levels down gives a 100% to the underdog (600xp).
Lane minions
- Ranged: 60+2/min
- Caster: 62+1.8/min
- Melee: 70+1/min
For Mercenaries individual xp:
- Knight 'melee' 50+2/min
- Seige Giant 50+2/min
For objective creatures:
- Garden of Terror:
- Shamblers : 60+2/min
- Terrors: 350+12/min
- Blackhearts Bay Pirates: 49+1/min
Experience you gain is influenced by your team's level versus the enemy team's level. There's an anti-snowball curve that helps teams catch up or prevents teams from pulling too far ahead.
But, if you're assuming same-level teams:
Heroes are worth 300 XP at level 1 + 50 XP for each level.
You can verify this in-game. With the May 2015 patch, the +XP floating text shows how much XP you earned from hero kills.
Ahli on reddit has datamined minion and mercenary stats.
For standard minions:
Footmen are worth 70 XP + ~1.2 XP/minute
Wizards are worth 62 XP + ~1.8 XP/minute
Archers are worth 60 XP + 2 XP/minute
Waves come in a set of 3 footmen, 1 wizard, 3 archers.
A full wave of minions at the start of the game is thus 452 XP. Compare that to a hero kill of 300 XP at level 1 and you'll see why many people prefer to lane rather than fight in the early game.
Assuming you gain roughly 1 level a minute, at level 10 you're looking at 566 XP for a minion wave versus 750 XP for a hero kill. Correspondingly, later in the game there is a much higher focus on hero killing and team fights over simple laning.
The break-even point for minion vs hero XP seems to be about level 5, but most characters don't have the talents to effectively burst and destroy heroes quite so early. Also remember that the exact values depend on your team's level difference.