Yep, $30-50 isn't gonna get you much aside from a "deluxe" Halloween costume. Considering a normal unsculpted adult jumpsuit can run you close to $30, you'd be hard pressed to find a muscle suit for cheap. Even fan made replica suits can cost hundreds of dollars, considering all of the tasks and materials involved in making one: sculpting a positive, making molds, casting in rubber or foam, trimming, and painting.
One thing you could do is get a muscled costume, then reinforce it. Whether that means coating it in a stronger outer surface, or reinforcing it underneath with more foam or another material, you can build up a standard Halloween costume into what you need.
Another option would be to make it yourself. If your character has an outer shirt, you could take upholstery foam, carve it into the muscles you need, and glue it to an undershirt that you put on before the costume's final layer.