What you need is a feature of our upcoming version 7.0 called Product Combinations. Each combination has in fact its own price, not a modifier. It allso appears as a separate product - has its own SKU, stock control, image etc. Henceworth it does what you ask for. This feature is available for beta testing, which is described here:
http://www.ecwid.com/forums/showthread.php?t=4398