Upload & Sell: Off
I use a RRS B87-B Portrait Perfect package (medium) with my 500/4L IS and Better Beamer. It's not exactly what I'd call 'economical', but with the purchase of a second flash mount pedestal, it lets you mount two flash at nearly opposite sides of the lens, which is great for macro. Mine's slightly older than the current model, link below, but works just great. You might find one used here on FM or on eBay.
The B87-B is great ... considering what one pays for the camera body, lenses, and flashes this is not that expensive. From an engineering and manufacturing perspective I can tell you that the parts are worth the price tag - and just in case, the generic RRS components have a really good resale value due to the excellent quality.