Sort order in product option dropdown

I'm setting up a product so that the store owner can do transactions for orders taken on the phone. I'm doing this in Pounds Stering (GBP) using Options for 100s, 10s and 1s as follows:

Hundreds: zero, £100, £200, £300
Tens: zero, £10, £20 up to £90
Single zero, £1, £2, up to £9

This works fine but the items in each dropdown are in a seemingly random order, for example...
Tens dropdown shows 50, 40, 30, 20, 80, 10, 70, zero, 60, 90.
Single dropdown shows 6, 5, 4, 3, 8, 2, 8, 1, 7, zero.

Is there any way of sorting these dropdowns so that the values run lowest-to-highest?

If this isn't clear, see http://www.artisan20.co.uk/index.php?ro ... duct_id=51

To update this, the order of the dropdown items isn't fixed, it changes every time I visit the page. I've also tried removing the zero option from one of them but that doesn't make any difference (that option has to be there to allow a zero value anyway, for example if the required amount is £203).