On average, 17 cs = one kill. In lower elos, the average farm per 20 minutes is 100, where in higher elos is can be anywhere from 150-220. If your lane opponent has 2 kills up on you, but has been csing poorly, you might still be ahead of him. BUT! He probably will still have the experience ahead of you, so you will have to wait a little in lane before you hit the same level he does.
Let's say you had 120 cs at 20 minutes, you are 0-2-1, and the enemy top laner is 2-1-0 with 80 cs at 20 minutes.
120 (your cs) - 80 (their cs) = 40 cs ahead.
40/17 (the amount of cs that equals a kill) = 2.352
Look at that! You have MORE gold than your opponent! BUT WAIT! THERE'S MORE!
You see that assist you have? Since you got it during a gank against the top laner, you just gain +150 gold, which is half a kill!
2.352 + .500 = 2.852.
In kill terms, you have .852 more gold in kills than your opponent. Let's translate that into numbers.
2.852 x 300 = 855.882 gold from your cs + assist lead.
Since you gain 7 gold per 5 seconds on Summoner's Rift, let's translate that into how much base gold both of you have.
60 / 5 = 12
7 x 12 = 84 gold per minute.
84 x 20 = 1680 gold at 20 minutes.
So both of you have 1680 to start with at 20. Let's see how much gold you have.
1680 + 855.882 = 2552.882 gold.
Your opponent, on the other hand, has only 600 gold from kills, so let's see his budget.
1680 + 600 = 2280 gold.
Let's see the difference.
2552.882 - 2280 = 272.882 gold.
So, just by focusing on farming, you were able to sustain not only an equal term of gold, but a higher total! You were able to farm to the point where you might as well had been 3-2-1 instead of 0-2-1, in terms of kill gold. You are actually AHEAD of your opponent, simply by farming! Again, you are still behind in experience, so you can make up for that by being in lane, but you will still offer more in a teamfight if you are both the same level.
It just goes to show how important farming is, and it can single handedly get you way more fed than kills will in a game.