UK exports to the EU are some £133 billion a year. Average WTO tariff rates, which is what we would be stuck at sans Single Market membership, are around 3%. We would thus save £4 billion. This is less than the £8 billion cost. This is not worth it.
Now relax our conditions a little. The £4 billion is being saved by EU consumers. Or, if you want to consider elasticities of demand and all that, anything up to £4 billion is saved by UK exporters. At a cost of £8 billion to UK taxpayers. This is still not worth it.