Anyone run into any weird quirks with the system?

Initially it looks like the cost to upgrade the specific item is fixed, so it might be better to add only the lowest ones to the highest ones.

Example:
Shield Necklace +8

Add another +8 - goes to level 10 for a cost of 180
Add a +1 instead - same - goes to level 10 for a cost of 180

Is this consistent with what you guys are seeing?

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Additional note: Just like upgrading the girls, it is significantly cheaper to upgrade initially than to try and upgrade it a second time. So pour all you can into that first upgrade to save gold!