I just bought the cheapest digital one at Harbor Freight. Works for me. Mine is several years old. This looks like the newer version. http://www.harborfreight.com/1000-gram-digital-scale-60332.html $10.99
Most digital scales (even the inexpensive ones) are accurate enough for hobby use. I would avoid used scales as you do not know what they might have been used for prior, so buy new.
I have this one: https://www.amazon.com/gp/product/B012IJCI1Y It costs around $30. It has a 50g weight limit, accurate to .001g, and comes with calibration weights. I primarily use it to weigh ancients and it works great for pretty much anything except the largest bronzes. It's also very repeatable. Great little machine.
I know a few people who used to use HF scales for said purpose, although I cannot confirm it was the same one, nor have I even looked at it. That said, if you do take this route, Harbor Freight has 20% off coupons (check online) that you can use to save even more.
To be relevant for coins, a scale should have a resolution of 0.01g. There are coins for which 0.1g is outside the allowable weight tolerance, making that resolution useless for considering authenticity.
I bought a Chinese one that goes to 10g with 1mg resolution for ~$17. I think I over paid but it was "guaranteed accurate". From my tests it seems to be accurate within 2mg.
When I bought the 10g scale, I also bought a 2kg scale accurate to .1g. The 10g was for coins, while the 2kg was for small packages. I think both together were <$30.
