Because the Welsh Government have released the figures!
"Taken together, the £243m loss in rural funding and the £772m shortfall in EU structural funds (table 2) add up to just more than £1bn. Applying the same inflation adjustment to rural funding as to the structural funds, the overall shortfall to the Welsh budget is more than £1.1bn.
These figures do not take into account the losses of funding from our access to other EU funding programmes, including Erasmus, Horizon and European Territorial Co-operation which have not been fully replaced by the UK Government."