I want to purchase a copy of Elements for a separate system, and I already own a copy of Pro 10.5. Do I qualify for any discount or do I need to pay $99 for it? TIA
You need to pay full price.
Your Pro licence will allow you to use Elements, as I understand it, but you would need to move your eLicenser between machines to do so. If I understand you correctly, that is not what you want to do, though.