Over designed and based on 8′ long x 2, 6′ wide x 2 with 4 x 2.5′ risers.
4″x4″x16′ non-treated (8′ x 2) $15
4″x4″x12′ non=treated (6′ x 2) $12
4″x4″x10′ non-treated (2.5′ x 4) $ 10
2″x4″x8′ non-treated x 2 $ 6
BC4 Post Cap Braces x 8 $35
#10 x 1.5″ Hex Head Screws (SKU 479710) $12
3″ Steel Casters (300lbs x 4 SKU 194425) x 4 $22
GRAND TOTAL $112

I would go with those Home Depot caster for long term. Go with these.
http://www.73ghia.com/?p=1788