Hidden spy cameras are lawful if the purpose is lawful. If you use these cameras to stop theft and vandalism, then it is okay. If you're using these in a bathroom, that that is immoral.
Hidden cameras are only illegal in the US if the tape has an audio track to go along with it. There are, however, a few states that have adopted laws that ban hidden cameras all together.
In general, video surveillance is perfectly lawful. However, if cameras are used in areas (such as bathrooms and bedrooms), it may constitute an invasion of privacy and therefore be unlawful.
There are a variety of hidden spy cameras, built into various household objects. The kind of object it's built into and the technology quality determines the pricing. A hidden spy camera can be...