I suspect that that notion is not supported by facts.

Referencing a chain parts store pricing, the price for full strength national brand is $12.99, 50/50 is $10.99. Store brand is $11.99 and $9.99, respectively.

Using the store brand for reference, the 50/50 gets you $6 worth of coolant (at the full strength price) and $4 worth of distilled water (1/2 gallon). That's $8 per gallon for distilled water.