The argument that the UK would save the sum of money on the side of the bus is analogous to a business that pays a fee of say £20/day for the privilege of selling goods at market. Each day the business sells £1,000 of goods at the market, making a profit on those goods of £500.
The owner of the business decides that they will save £20/day by not paying to attend the market.
Brexiteers will claim that the business has saved £20/day, others will realise that the business has lost £500 in profit/day (that is the position UK Plc is in as a result of Brexit).