I paid $600 for a 50" Rotavator J model, but I had first borrowed it and couldn't return it in time but that's a long story.
The tine were worn badly, but it tilled just fine. I replaced all 28 blades for $225 and it doesn't seem to till any better - still just fine.
Parts are available. Pull the back of the gearbox off and if it and the drive chain are in good condition, it should be worth $400 if the width is ok with you. Me........I'd like to have a tiller that is as wide or wider than my tires so you don't have tire marks in the field.