I've checked a couple of cheaper ones in th epast and they aren't all that bad.
Top notch ones will be certified to 2 or 3% accuracy, the worst I saw was about 10% over-reading at most, generally around 5% optimistic (i.e. the scale showed a higher figure than the wrench clicked at, which is the generally safer direction).
10% might sound a lot but if it's actually 22.5Nm when it ought to be 25Nm it's not going to make any difference in most instances, unless you intend to do big-end bolts etc.
Best cheap ones are Draper IMO, you can get a 10-80Nm 3/8" drive one for around £20. As an example, look
here .
Have a look
here too.