Depends on the language you want it coded in. Something in general use like VB is less expensive. A good place to look for programmers is dice.com
If your trading platform has the ability to execute encapsulated code and the system you want to implement can be defined by modeling then there is a much faster and cheaper alternative. Many of the general purpose data mining applications have the ability to take historical examples along with your desired goal and turn them into a predictive model. The results can be exported to VB, C++, Java, XML, etc to be used by another application (your trading platform).
Since the software does the heavy lifting, the human effort is limited to defining the inputs and the goal you want to predict, so the human hours are a fraction of the cost of having a programmer code a complex rule based system. This is the approach I went to after spending dozens or hundreds of hours programming ideas only to find they didn't work as well as I thought or they stopped working after the market evolved its patterns over time. With a model you can remodel the market as it evolves or even weekly as opposed to having a static programmed system.
The actual run time to generate a model for most commercial grade predictive applications is less than 24 hours. This may compare with weeks and weeks of programming and testing time for a complex trading system.