Thanks for the quick replies.
wbport: there is a tally for a cost as a result of the combination of boxes that have been selected - having to make your selection then click to 'update' the total is not as elegant as having it update with each change.
xelawho: sorry, I'll explain properly.
(this is just the model, the use case is slightly different but easier to keep it simple).
There's a column of X checkboxes. Each one is a product with a cost. The user can select any combination of products (up to a defined limit Z). Initially there's 1 text field that's blank.
A checkbox is clicked, the price is put into the empty field. Second box is clicked, a new field is added and the price goes in. So on and so on. If a user unselects a box, that field is removed. Ideally if unselecting everything will go back to initial state with 1 empty field.
The adding/removing fields is icing if it's not overly complex as I'm quite happy to just have all the input fields there at the beginning but empty, but it would make it look that little bit more elegant.
I hope that makes it more clear. Appreciate this raises quite a few 'why...' questions - the use case does make sense, but it's somewhat more complex and doesn't contribute anything relevant as to what I'm trying to do.