It is hard to even consider comparing. There are other models that cost a lot less, that can easily run this pre. This battery http://www.batterygeek.net/Batterygeek_15_21_118_External_Laptop_Battery_p/15-21-118_batterygeek.htm costs $169, one time. It weighs roughly 1.7 lbs. Twelve D cells, weigh almost four pounds, and I am sure would hit that mark, with the weight of the sled included. 12 D Cells, at Costco cost at least $10. So, after using the battery for more than 15 times or so, it is not only cost efficient, but weighs well less than half as much, as well as being about a third of the size. Oh, and you forgot to mention that the cost of your plastic battery sled is $225-250, which is more than the entire most expensive battery option from batterygeek, and then you still have to buy the batteries. I really don't know who you sell those to.
To make easier comparison lets consider solution
for ONLY SD744T deck using the largest Batterygeek $499 200 watt (but ~170 watt output at 85% efficiency) model weighting 4.4 pounds that would need $170 -$225 every 2-3 years for replacement battery to maintain capacity.
For
apples to apples power capacity comparison
Compare the above model directly with BD-12 at 4 pounds, higher 200 watt power output per charge, and costing about half that of the Batterygeek (nearest equivalent) product.
The BD-12 cost difference buys you 350+ hours of worth of operating time using D cells, so at least a few years worth for most people. AND the BD-12 is still LIKE NEW after a few years, and will remain this way for at least 12 years! With the Batterygeek product, you would have to REPLACE THE ~$190 BATTERY EVERY 2-3 YEARS
regardless of using it or not to have this device working as new.
With the BD-12 new condition is always ONLY having new D batteries, and ONLY for each period of 20+ hours of operating time, so cost of BD-12 makes EVEN MORE sense after a few years of owning as additional maintenance cost is ONLY what YOU NEED at the time.
This same type of 'apples to apples' comparison to the 'controversial' C/D cell solution is possible with R-4(PRO), MT, and SD700 deck models verses the equivalent (output power) of a Batterygeek type product.
And YES, my 'controversial' battery solutions for MT, R4 (PRO), and SD700 decks ARE well accepted and appreciated.
These products sell mostly to nature, documentary, and film sound professional recordists who sometimes spend a bit of time outside the power grids, and/or require the longer running time, higher reliability (even after many years of field use) than is possible with Batterygeek type solutions.
I think you'll find the C/D alkaline cell solutions make a lot more sense with direct comparisons with having lower weight, more output power, more reliable with 100% consistent power, and costing far less over a few years of ownership than the existing (Batterygeek type) rechargeable solutions.