The first thing that you can do is charge by the hour. This is great if you are going to be working on a long job. As for the exact amount that you should charge, it would be best to look at what everyone else is doing and go with that, because chances are that the client has already looked into several different consulting firms and already know the standard fee for consulting jobs of this nature.
If you are going to be working on a job that might only last for a few days, then you could try to charge a fee for the entire job overall. Now there's going to be one problem with this, hopefully it's a problem that you can actually avoid. You might assume that the job will only take a few days, and in fact you might be told by the company that it will only last for that time. Then the job might take a lot longer, which is going to be a serious problem if you set a price for the job that only covered the two days. To avoid this problem either use the hourly model or make sure you tell the client that extra time is going to cost.
Another pricing model that you can use is one where you charge based on the outcome of your job. This means that you will only be paid a decent amount if you deliver what you promised. Now the problem with this pricing model is that you may not see your results for as long as six months. Even if you do, you have no way of telling if the results are accurate, or if they have been modified by the company. It is best to stay away from this type of pricing model because it is just too unpredictable in consulting jobs.
It's important that you make your client sign a contract for your services. As long as there is a legal contract they cannot back out, and they will be forced to pay you for your services. Always make sure that all of your paperwork is in order, we cannot stress this enough.
These are just a few models that you can use in your consultant jobs. Consultant careers and consulting jobs can be very complicated but they can also be very rewarding so long as you do everything right. In the early years you'll probably make some mistakes, but with experience you may find that you enjoy it very much. Take the advice of others, and make sure that you are pricing yourself accurately. Remember, you are worth quite a bit of money, because it's not just anyone who can participate in consultant careers!