Hello Andy,
Check the Price Formula extension for Magento. It allows creating mathematical formula for the product price with sub-conditions. For instance, let's say the first dropdown has sku "myqty", other options have skus "option1", "option2", .. etc. you can create a formula like this:
IF ({myqty} == 25) Price = ( {option1.price} + {option2.price} + {option3.price} + {option4.price} ) * 0.9
ELSE IF ({myqty} == 50) Price = ( {option1.price} + {option2.price} + {option3.price} + {option4.price} ) * 0.8
ELSE IF ({myqty} == 100) Price = ( {option1.price} + {option2.price} + {option3.price} + {option4.price} ) * 0.7
...etc.
And all this via backend. No coding needed.
Hope it will be of help
Roman
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