My lvl 11 MDM mana decay reduction of 6.4% is worth 360,000 a day. In addition I collect about 18,000 per day by collecting 5 times. Thus the value to me is about 378,000 mana per day. That's about 12,600 per square per day. (378,000/30) (I have about 10M of mana in stock).
My lvl 11 DA I gets me12,000 per Enchantment usage. Thus, to make the mana benefit even with the DA I need to use 31.5 enchantments a day to reach 378,000, the even point in my scenario. But since it takes up only 20 squares it's mana value to me is 18,900 per square per day. Adjusting then, the number of enchantments needed to match the per square benefit of the MDM vs DA, I reduce my enchantment needs by the difference between daily per square output. Since 12,600 is 2/3 of 18,900, and 2/3 of 31.5 is 21, that's the number of daily enchantments I'd have to use to get the same benefit per square from the DA as the MDM.
All of which leads to two questions: 1) Do I use 21 enchantments per day?, and 2) Is there a better per square set of mana productions that will automatically collect (i.e. not lose) 360,000 mana per day and take up less space than the MDM or a set that, with 5 collections per day, using the same space, achieve a total of 378,000 per day?
In my case, the answer to the first question is: yes. I consistently run 5 MM per day and about 30 EE, which is probably why my mana is up to 10 million and climbing.
In my case the answer to the second question, because my mana is so high, is no. No combination of my available mana production buildings come close to the 12,600 per square per day mana benefit as my MDM provides.
But of course, the amount of mana one has on hand plays a big part of how much benefit the MDM has. If my mana on hand goes down to 2,000,000 it saves me 72,000 per day. With the 18,000 collection bonus, I have a 90,000 daily mana benefit, or per day per square or 125 per square per hour. Is there any set of mana production buildings that get that production rate and take up less room? The only things do in my city are the mana plants which produces 200 per hr per square and the mana huts at 135. But since I rarely go below 5 million in mana in stock as my stock grows the MDM becomes a bigger benefit. At 5,000,000 the benefit per square is about 285 per square per hour, higher than anything else I have in stock.
From this you should, I think, get two things. First, it depends on how much mana you have or want to have in stock. Anything below about 2 million starts dropping the value of the MDM, anything above that raises it's value. Since I like keeping about 5 million in stock as my "minimum" the MDM is very much worth the space to me. I virtually never run out of mana, and that, in either chapter 16 or 17, if I remember, was a god-send as one of those chapters takes a lot of mana.
AJ