for 35% of the cost of the battery geek, you can get this 7200mAh:
Maybe a good option for the Tascam 680. It is designed to power LCD screens in the wild. For the price it looks like a decent deal.
http://www.adorama.com/MHMLCDB26.html?utm_source=gbase&utm_medium=Shopping%20Site&utm_campaign=Other&utm_term=Other
115 Watt-hours for $200 comes out to about $1.75/Watt-hour
26 Watt-hours for $70 comes out to about $2.70/Watt-hour
So one battery costs less simply because it has about 1/4 the total energy of the other but it is actually more expensive per unit of energy. You can't just compare the capacity because a unit like the DR-680 has a switching regulator at the input and excess voltage is not wasted. Also, both of those batteries have a regulated output which isn't necessary with the '680 because of said regulator. So you're wasting around 8-20% of the energy contained in the cells dropping from 14.8V to 12V.
Also, I think the 7.4 amp hour figure came from adding up the capacity of all the cells instead of only adding up the cells/batteries wired in parallel. 7.4Ah x 12V == 86Wh but they only claim 26 watt/hours.
Capacity when new is only part of the story. Batteries don't last forever and can't be recharged an infinite number of times. How long and how many times depends on the chemistry. A LFP battery like the ones on the page below have 5-10 times greater cycle and calendar life, so over the lifetime of the battery it is much, much less expensive than any regular Li-Ion or Li-Poly battery like the two above. Even if you consider that it doesn't come with a charger. Also LFP is much safer because if damaged or shorted it won't burst into crazy unstoppable fire that will destroy your gear, fill the air with noxious smoke, and get tapers banned from the venue.
http://www.batteryspace.com/12-8v-lifepo4-battery-packs-from-3.3ah-to-6ah.aspxThe above batteries are even cheaper per Watt-hour than the batteries you guys are talking about, one is only $1.04/Wh. Even when you don't consider calendar life, cycle life, or safety it is a better deal and remains so when you factor in a connectors and a charger. It is definitely time to switch to LiFePO4 batteries!