The complexity of working on a colorization depends on how many people (or objects) depicted in the photo.
The more people, objects or small parts, the harder the work. Samples and approximate price you can see below.
The price can be increased if the photo needs significant restoration (damage, dust, scratches).
Depending on the complexity, I can offer three price categories:
Simple colorization: from $30
Medium colorization: from $40
Complex colorization: from $50