Assuming the lights had originally been wired correctly and were working properly, usually the problem can be found and fixed by an electrician, well trained in trouble shooting, in one to four hours.
In troubleshooting it has been my experience that the majority of the time is in finding the root cause of the problem. Often once the problem is found a connection needs to be tightened, a switch replaced etc. So, determining the time and cost before finding the actual problem is not possible.
The above answer is based on the following assumptions:
• You have already replaced the light bulbs.
• The lights that need replacing are not fluorescent lights (as they may need ballast…)