I think Metal Rescue may be similar to Evapo-Rust, which I use all the time. Dunk an item overnight and the rust is gone. Some things come out looking surprisingly close to new condition. Evapo-Rust is the same price range you mentioned -- about $23 per gallon from Advance Auto Parts.

For bigger items like frames, I use oxalic acid instead of Evapo-Rust due to the cost. A small tub of oxalic acid crystals sold as Savogran brand "wood bleach" sells for $8 at a local hardware store. Mixed with water, it'll fill a kids' wading pool. Dunk items overnight to remove rust, rinse well, and dry.