How to Compute fields on the fly
This sounds like cashier functionality. There has been some effort for D6, see Drupal POS (Point of Sales) System. I haven't used this.

Ideally, this would be done with JavaScript (I have once done it for a non-Drupal system).

A cheap workaround would be the Save &amp; Edit module, where you just save the node and continue editing it, effectively updating the calculations. On demand, not automatically.

A smoother workaround would be updating (on demand) the form with ajax. Ajax module  has no D7 version, but in the issue queue is mentioned this sandbox project which supposedly can ajaxify the node edit form:
Asaf (ajax submit for any form).

You need one extra module for actually displaying Computed field on node edit form: Field extra widgets. Select the widget "read only".