SAD1 or C88A's as we in the UK call it, 12 pages of questions. Comes with a free
8 page continuation sheet . Seems completed copies are sent to different places. Presumably this would need to be completed for every sale and shipment of goods to and from the EU.
Any HMRC estimate of costs for business will have come from businesses themselves. I am aware of surveys they have done in the past on costs incurred in meeting HMRC obligations. Given these forms already exist are are used by businesses (for sales outside the EU) I see no reason why the cost estimates would not be accurate.