Some cards aren't as good for upgrading as others, but you get a choice of what to upgrade and when. Pillars, for instance, are a great thing to upgrade (not sure why they are cheapest on your list...) because they are nearly impossible to win upgraded via FG or half blood, and they are able to go into any deck that uses a specific element, so they count toward more than, say, a FFQ that will go only into a certain kind of deck.
Others are not so good for various reasons. Something like a fallen druid can make great mutants for you, but at the same time, they lose their CC abilities (unless mixed with another card, and even then, its shaky) and so, while its a good upgrade, not something that you will want in every case. Or a rare, as you mention, like morning glory, gets only +1 damage, not a big deal (only 14% more damage for all that money!)
Others can be downright bad to upgrade. A fire nymph is more likely to be useful with the slightly cheaper, slightly weaker effect. It is something you likely don't want to upgrade.
But, if you were paying attention, I just showed an example of a super rare card that you wouldn't pay 1500 to upgrade, let alone more, a rare card that gains less than most other cards, a common card that gains a lot in many circumstances, but isn't always going to be worth it, and mention that upgraded pillars are maybe the best investment, a full 180 from your suggested prices.
My conclusion, just leave it as it is. 1500 is a lot, but not so much that you won't make it eventually, and its better if its up to each person what value to put on upgrades, rather than pushing a formula that could make it even worse.
