I've had errors like this occur before. Usually, it happens when you have two different costs associated with the same unit. For example, if in the unit tab, Obelisk costs 100 minerals, and an charges 200 minerals for Obelisk, there is probably an issue.
Hope that helps.

A melee mod - The A-move stops here.
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ElBushido Regular ShmoeDuplicated Obelisk and changed the cost to 100. This 1 will upgrade from photon cannon. The problem is that the button showes that it costs -50 minerals(i tested by setting the units cost to 0 then the button showes -150) Any idea whats happening?
Interestinly tho I use the same morph ability (I actually duplicated this for the Plasma Tower and changed the button name and such) for a pylon that upgrades from a (weaker) pylon and i have no such button problem with it...Thought this might help a bit to narrow the problem source..
cost 0 tech cost(morphAbil) 0
cost 0 tech cost(morphAbil) 100
Pylon shows upgrade cost ...