Maybe we're talking about something other than the RL-550. The caliber conversion kits are only $49.95 and an additional toolhead is $26.95. Those are really all you need if you already have dies. The quick change kit is for folks looking for convenience. They come with a toolhead stand and separate powder measure.
You can pick up these items 2nd hand....typically $25-30 for a caliber conversion kit and $10 for toolheads.
I swap my powder measure around between toolheads, as it only takes 30 seconds or so. Similar to the Hornady system.
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XL650/750