5 mls of water for 30 minutes work doesn't sound great - 5 mls of alcohol, on the other hand ......
that would be a little bit better! You wouldn't waste it on the mirror though if you had that kind of distillation going ok.
back on topic - my local Super Cheap auto accessories place has 4 litres of distilled or demineralised water (not sure which, but it cleans my mirror ok) for less than $4. I haven't seen it cheaper anywhere so far.
Don't use "spring" water - read the label carefully, they are full of various kinds of minerals, etc, and the amunts and types vary hugely from brand to brand.