I don't think you'll find 5mm for less than the 10mm because it's down to the economics - the 10mm is used far more often as the plastic panels you are referring to are almost always sold in the 10mm variety.
Tbh the 5mm isn't worth the cost saving and you can compress it with your fingers - the vertical ribs are too pliable. Even if the 10mm wasn't that much cheaper I'd still not recommend the 5mm, unless the thickness was paramount in limited area.
info coming from just under 10 years experience in the trade / domestic plastics trade.