The League of Legends Wiki lists all of this. A table for the rewards for kills is shown in their Kill article, which I will mock-up below. Note that as of V126.96.36.199, the bounty values were reworked to increase the effect of each kill or death. The table shown below reflects the current bounty rewards as of said patch.
K/D Count = Killstreak/Deathstreak count for the champion slain
K Reward = Improved reward for killing a champion with that kill streak
D Reward = Reduced reward for killing a champion who is getting hosed
K/D Count | K Reward | D Reward
1 | 300 | 300
2 | 360 | 260
3 | 432 | 182
4 | 500 | 127
The Kill article notes that a first blood has a +100 gold bonus, so the first blood will always be +400 (as this question leads me to believe that executions don't increment your death streak).
The other point to take home is that multi-kills and aces do not provide any bonus. Likewise, getting a killstreak doesn't get you any additional reward, only the person who finally takes you down.
The article for Assist states that there is a pot of 70% of the kill bounty that is split between all champions that provided an assist for a particular takedown.