Hi Darren, just my 2cents as all regulars here are actually real experts (much more than I am) on Heuer boxes.
Referring to the the most sought-after and famous Heuer boxes (the red one with the chequered flag all around the lower half), main reason why they are so hard to come by is that they were made of cardboard and so most of them have not withstood the proof of time.
For the same scarcity reason, when they do show up they're extremely pricey and also those in not so good conditions fetch incredibly high sums.
Earlier boxes are even rarer for, shall we say, historical reasons whereas sturdier, later ones (e.g. the late '70's - early '80's blue ones) were probably discarded by their respective owners at the time and are not so 'fashionable' as the red ones, so probably they don't tend to end up on the Bay. Still I am quite puzzled myself for the scarcity of the blue plastic ones, any idea anyone?
Arno Haslinger's magnificent book on Heuer chronos has a whole paragraph dedicated to boxes, where you can find a lot more.
Hope this helps.
Cheers,
Fabrizio