A better Subject would be something like "How to split a string". Many people ignore posts with a meaningless Subject.
To answer your question:
"... I have used checkboxes in a form to create an array... "
You didn't create an array, you created a string. You could modify the form to create an array, or you could split the string to create an array.
"...split the contents of the array, so that I just simply have the costs in a new array..."
Your example shows the costs in another string. I might be able to help, but I don't know what you are starting out with or what you want to end up with.
