@Luca Lu: Thanks for your reply. I agree that it's a feature that would be great to have within Shopify -- I've reimplemented this on my own store using a hodgepodge of Javascript code adding and removing differently-priced variants :-)
I took a quick look at https://apps.shopify.com/quantity-breaks and it seemed interesting, but I preferred to recreate it from scratch. You can see it in action on my store (French-only for now): http://baltrakon.com/ when adding multiple t-shirts.
@Jason: Thanks, I might just do that. :-) Do you think that this is the best forum for this type of question? Perhaps a more general Shopify forum (not one related to Apps specifically) would be better? Cheers
Greg