Let us put in in context.
This is making it so that you get 200 gold + 1 gold for every creep killed so in a long game (should long games ever return) when you get to 250 CS this item has payed for it's cost while also giving you 5 ADand 3 health restored on hit which no longer is taking up an item slot once you get the first 100 CS. And yes if you were to get 600 CS in a game for some reason you would in effect have a net gain of 350 gold from this passive.
On the other hand you have doran's blade. When you reach the point where you would be selling a doran's blade which in a long enough game will happen you only get 180 gold back for it.
Once you get to that 100 CS threshold this version of Cull is all around going be have been a better investment in the long run the doran's blade unless you were either killed because of the lack of health or barely didn't get a kill on an enemy champ within like 30 health of them dying.
In reality this would make Cull an item to consider in lanes that are most likely to be a major farm fest such as Ashe and Soraka VS Janna and Ezreal.
It is a decision that you have to calculate for yourself based on your ability to CS and what is likely to actually happen. As such I do not expect it to be a good item for bronze 5 players who are not good at CSing but for players who are good at CSing this item might be a worthwhile investment with real consequences be them good or bad for choosing it.
The idea is not to make Cull all around better then Doran's blade but to have a clear reason to be bought over Doran's blade in some situations which it is somewhat lacking in it's current live form.