I did a bit of kitchen scale research a while ago, so I thought I'd write what I discovered in case someone finds it useful. Sorry about the length (inexperienced poster warning). TL;DR buy a lab scale without auto zero tracking).
The price depends on the number of scale divisions. A scale with a capacity of 1kg in 1g increments can show 1000 steps. So can a scale with a 100g capacity with 0.1g precision, or 10kg capacity with 10g precision. (This is just examples; scales use more levels internally.) The scales use cushion (load cell) that is deformed by pressure, and the deformation is what is measured. All that is needed to change precision is to use a cushion with a different firmness, but increasing the range requires more sensitive electronics (and a higher quality cushion). That makes it cheaper to buy two scales with different precision if both precision and capacity is wanted (but not at the same time).
Most (all?) scales have overload protection, as excessive force would permanently deform the cushion. Even with this protection, overloading is still a problem. MyWeigh cites it as the most common source of fatal error
. Even if the scale doesn't die from it, accuracy might degrade. Be careful with them, and never store anything on top of them.
It is normal for measured values to drift. This happens due to things that affect the internals (temperature, humidity, voltage fluctuations) and the thing being measured (evaporation, dust, air currents). I guess temperature affects the elasticity of the load cell. To quote the manual of my scale, it says that it requires 30 minutes "worming up". (It also comes with an AC adaptor to supply power from "mines".) The feature to combat this drift is generally called auto zero tracking (AZT).
AZT means that small measurement changes gets rounded off, and the tare value is updated. Any change in weight that too small to show on the display is discarded, which means the display will show a stable value even though the measured value changes over time. If the drift is faster than the scale updates the display, you can see the value change a little or run off a lot (which is what happened to an earlier poster in this thread). AZT may be fine if you're weighing discrete items (such as apples) or counting nails, but makes the scale useless for powders and slowly drizzling liquids.
When buying a scale, I made it a requirement that it should be possible to disable AZT. I keep it off all the time. Some people might find it disconcerting that the values changes every now and then, but I prefer that the scale doesn't hide these errors from me. The measurement error when adding items incrementally is reduced, clouds of powders settling on the surface isn't rounded off, and I can drizzle liquids as slowly as I want.
If you don't find a scale without AZT, there are some things that will reduce its effects. Higher precision is good, as it leads to smaller rounding errors. The second thing to look for is the display update rate. The faster the display updates, the more frequently it can introduce rounding errors. Some scales allow setting the AZT filtering level. Reducing it will make the display update less often, but you'll get better accuracy.
Kitchen scales falls into the bigger category bench scales. Don't restrict yourself to scales with "kitchen" in the name, as difference is just in visual appearance and advertising. If you want a quality scale, I recommend looking at the ones designed for laboratory use. Unlike kitchen scales, they generally come with a large number of features and an RS232 port. They are made in series with varying capacities and precision, but with identical design and features. Find a manufacturer that has the settings you need (no AZT or auto shut-off timer!) and pick the desired combination of capacity, precision, accurace and price. Stores usually only carry on or two from a series, and generally not the cheapest ones. I guess more research on where to buy those is needed.
Apart from the stove, I think the scale is the appliance I use the most in my kitchen. It's a lot easier than using measuring cups and spoons (and less to clean up afterwards). I've tried confining it in a cupboard, but it inevitably finds a way to escape within a day. I've used it twice today, but I haven't started making dinner yet. It has one downside: it beeps when turned on or off. I'm considering surgery. Or maybe I should just keep it on a permanent spot in the kitchen, always plugged in and all wormed up using the power from mines...