Lightweight usually means forged.
Regular 20" rims usually weigh around 35lbs. Forged rims should weigh around 24lbs.
Reasonably priced forged rims are hard to find for less that $650 ea., and often run as high as $1k ea, or more.
When I was shopping for 20in forged rims the best deal I could find was Forge-tec. A set of 4 wheels can be purchased from Moparsupercenter.com with tires for $2,619. The rim I picked weighs 24lbs. MOPAR