Shooting Illustrated has
an article on budget variable power AR optics. Three scopes it looks at are,
Firefield 1-6X24mm 1st Focal Plane Illuminated Riflescope
Weaver Kaspa 1.5-6X32mm
Leatherwood 1-4X24mm CMR Tactical
These are scopes, not RDS(red dot sights). RDS is what we usually think about when it comes to AR sighting system, but there are good scopes that can be used too.
From:
Shooting Illustrated