When you are entering a trip request you can use the trip estimator to see approximately how much this trip will cost (the school, the students,etc). The estimator section is toward the bottom of the trip request form (right above the acceptance of responsibility section)  and you need to have filled out the entire trip request before you use the estimator in order for the info to show up about your trip. The estimator section looks like this :

Once you have clicked to open the estimator section up it will look like this :

***Remember....this is only an Estimate. Not the final cost of your trip ***

At this time you can enter the venue cost into the box - for this sample trip I'm saying that it will cost $7.00 to get into the venue - In the number of substitutes box I'm going to say that I need to use 1 substitute on the day of my trip - If you have any other costs you will enter that amount in the other costs box (I didn't have any other costs so mine stayed at 0.00) - It will look like this with my figures added into the proper boxes :

At this time I'm going to click the Compute button - notice that the amounts have now been updated - the total cost of this trip will be $518.52  and the amount that the student needs to pay is $15.71 :