Sweet action. This is awesome, and I'm glad I didn't have to figure out how to do it.

A few things I want to point out:
In _calculate_price(), use {uc_product_options} WHERE nid = %d AND oid = %d. You can change the price of an option on a per-product basis.

Use uc_currency_format() instead of number_format() so the $ sign isn't hard-coded.

Do you know why there are so many AJAX requests for every selection that's made? It seems excessive, but I haven't dug deep enough yet to find out why it's happening.

Great job.