sorting of a price field

Hi,

I run a fairly simple query to show current records of stock using the
phrase 'sort by shop_price'.

Surprise (but really no surprise) this is sorted in ascii order rather
than shop_price order. So 110.00 comes before 20.00

I can't see any reference to changing this. What can I do, is this
perhaps to do with the field type, can it be changed?

Thanks
Anne