By the time you fulfill that condition, either 1) your opponent(s) are feeding or incapable of doing much to you, in which case you've pretty much won your lane and Cull becomes an Overkill item, or 2) you'll have already started Roaming, which by the time the 300-gold condition is met you've already gotten assists or kills on enemy Champions.
If you really want the gold that bad, get
,
, or
. The amount of Life you get from
won't help much when you've already started building other items because of the gold you got from fulfilling any of the 3 items' conditions. They even cost 100 gold less, meaning you can actually buy Potions (or a Refillable Potion) or Wards. The bonus of being able to build into
/
,
/
, and
/
far outweighs the benefits of such an item like Cull.
I just don't see the benefit of getting an item whose only use is incompletely refunding what you spent on it. Even if you're getting "400 plus minion farm", you'd still have more gold if you just spent nothing. Or you could get a couple Health Pots and save the rest for something more worthwhile like
.