Add 'Order at other distributor based on minimum price difference' cfr 'distributor is % lower'
Now it's only possible to order items at other distributors if a certain percentage is entered within 'Distributors' - 'If price from other distributor is % lower, select that distributor.'
But if for example you know the shipping cost per distributor (provide extra field at Distributor), you can calculate this in the 'if cheaper then' selection.
In this case you can make sure that the ordered item wil always be cheaper no matter where it's ordered.
If you enter an percentage, for example 5% cheaper, on items with an high cost, the shipping cost shouldn't be an issue, but if it's 5% cheaper on a product under 50€, your shipping cost will make it more expensive in the end...