Well, I don't know the specifics of what you are talking about from the information given...but in addition to Loup's comment you also need to look at upkeep cost.
Lets say you build 5 regular industrial buildings and they have 5 upkeep and produce 10 industry. So you are producing 50 industry for a cost of 75bc (been playing Eve too much lately..I almost said ISK) That's 1.5 bc per industry point.
Now you build one of your 10% bonus buildings, but it's upkeep is only 1. You are adding 5 industry for a cost of only 6bc, lowering your cost on this building to 1.2 bc per industry point, and your overall cost to 1.47 bc per industry point.
(all of that is assuming 100% spending of course. The difference is bigger if your spending is throttled back..and yes, those are theoretical numbers just to point you to something else to consider when looking at those bonus buildings.)
