The scale you should use depends on what distance you shoot at.
If you do not normally shoot past 5 to 600 yards, you will be fine with the accuracy of the $100 variety of reloading scales. In this price range they typically claim accuracy of +- .1 grains but in reality it's more like +- .2 grains. Even so, a total variation in powder charge of .4 grains will not result in enough of a velocity change to cause you any acuracy issues at such close range. The velocity fluctuations due to powder charge variation will result in about 2" of vertical variation at 600 yards.
If you are a long range shooter, that's another story.
A variation of .4 grains will result in a vertical shift of about 10" (give or take a bit) at 1000 yards - depending on case capacity.
If you like to shoot that far then you need to control your velocity better than a $100 scale can offer. At that point you will need to spend about $300 on a miligram scale. These are accurate to about .02 grains or about 1 kernel of 4350 powder. I suggest a Sartorius Element
http://www.itinscales.com/sartorius_element_lab_balances.htm
If you are considering an RCBS Chargemaster my advice is forget it, and get a miligram scale instead. The price is about the same, but the chargemaster cannot offer you 1 kernel accuracy on your powder charge.
If you are well heeled and can afford more, dont get a scale that is more sensetive than a miligram scale. It will drive you insane. Instead just get a better quality miligram scale as the next step up you will be over $1000 but they will last forever, never ever drift and are fast.