I have never seen the 42 watters in multiple packs... I have only seen them sold as singles... I have looked for multiple packs..
they range in price usually from 7.44 (target) to 9.47 (home depot)... I have seen them at home depot for 4 dollars and some change a few years back...
cheapest place to get mylar is in the camping section of any department store, in the form of emergency blankets - they're usually $2-3ea and 1-2 would cover your grow area.
it is however extremely thin and prone to ripping...if you can spare it, i'd go for panda film from a hydro store - it's around $3-4 a meter and is thicker and water-proof as an added bonus.