It all depends primarily on the level of the specialist. If you are looking for a junior-middle level employee, $150-500 will be enough. For more experienced candidates, you will most likely need a reward of $500 or more. And if you are looking for a highly qualified professional in demand on the market, the bonus should be at least $1000.

The main point is that the bigger the bonus, the more chances you have to hire the right person.