I found that the hidden cost that is "reported" (which obviously is untrue since the upgrade is still the cost that it was intended to be) is somehow connected to the actual unit itself.
For example, I tried upgrading a photon cannon (created from scratch, not the original Photon Cannon object) for 10 minerals to a stronger version, but the tooltip recorded a cost of -140. If I went into the original Photon Cannon which is still unedited and changed the cost to 150, the tooltip for my upgrade would appear accurately. (Of course this has no change in gameplay).
I'm not sure where the connection is, but this did fix my problem, which I think is the same one you have. I suspect it must be from a shared actor or something...
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I found that the hidden cost that is "reported" (which obviously is untrue since the upgrade is still the cost that it was intended to be) is somehow connected to the actual unit itself.
For example, I tried upgrading a photon cannon (created from scratch, not the original Photon Cannon object) for 10 minerals to a stronger version, but the tooltip recorded a cost of -140. If I went into the original Photon Cannon which is still unedited and changed the cost to 150, the tooltip for my upgrade would appear accurately. (Of course this has no change in gameplay).
I'm not sure where the connection is, but this did fix my problem, which I think is the same one you have. I suspect it must be from a shared actor or something...