If you have 6 M2 turrets, next turrets (M2)will be at 20% discount if they're available at your rank. If you have 6 M1 turrets, M2 will not be at a lower price. Your turrets need to be of the same modification.
It says that you have 6 turrents then can get 20% discount. http://tankionline.com/en/2015/09/growing-your-arsenal-is-now-easier/
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