This is absolutely untrue. Production cost may be fixed (or close to it), but there is an awful lot more that goes into the cost of a mini than production. In fact, production cost is probably well under half the overall cost; that's true for most consumer goods.
In the random-vs.-nonrandom scenario, the single biggest cost variable is the cost of unsold product. The cost of every unsold unit of product is a direct tax on the profitability of the item. In a randomization scheme, you only have to roll the dice on production numbers a few times a year. In this new scheme, WotC has to get the numbers right for each individual SKU--and it's more complicated, because the consumer response to each SKU might be different.
To account for this, WotC's product P&L must include a cost for unsold product, and that means their costs go up.
[To further complicate things, it's not only WotC that has to deal with this. The distributors and retailers also face an increased inventory risk and an associated cost. So at the end of the chain, the price you pay reflects this increased inventory risk three times over! Merric's law rears its ugly head!]
