There are a lot of different types of personal breathalyzers on the market; some are inexpensive and of low quality, some are expensive but are of higher quality, and many are in between.
A portable breathalyzer that the police use costs at least $500, but it is also generally more accurate and reliable than a personal breathalyzer (and larger), and its results can be used in court. Personal breathalyzers can cost as little as $10, but these tend to be novelty items and have very little accuracy. The personal breathalyzers that are truly useful in terms of their accuracy range in cost from $50 to $150 and range in size from that of a keychain device to one that can fit in your pocket.
As is true with most products, you generally get what you pay for!