Java Script for adding value to a text box if another box is checked

Hello, Java Script novice here. I've seen a few posts with this same question, but haven't quite gotten it to work yet. I have some fillable forms in which I would like an amount to populate in the corresponding text box when checked, but would like it to remain blank when unchecked. I would also like to have a total populate at the bottom depending on which boxes are checked, to be added to a base price. I was able to get it to populate the amount once it was checked, but then if I unchecked the box, the amount would stay in the field. I'm a complete beginner, so if I could be able to cut and paste a script that would really help. I have included one of the forms as a reference. Thank you in advance for any help you can give.