Real Life Example of Estimation

Was at the supermarket buying mama dishwashing liquid. Mega value 2x 1litre bottles going for $7.10 versus one 600ml plastic refill for $1.60. Which is better value?

4 packets of the refill would be 2.4litres which is more than the 2 bottles. Yet it would cost only $6.40. So it is definitely better value.