The keyboard and module don't require much power at all. The thing you have to figure out is how many amps your cube monitors take, both at peak, and in typical use. If the whole setup takes 3 amps (and I would be surprised if it were that much), then a lawn tractor battery (32 amp-hours continuous) and even a 500-watt sine wave inverter would have you covered.

The difference between this stuff and the FlashFish device, is that the tractor battery is a sealed lead-acid gel cell. They are durable, long-lasting, and comparatively cheap, but heavy. The FlashFish probably contains a Lithium-ion battery (think of an overgrown cell phone or digital camera battery.) They are much lighter for the same power capacity. Also more expensive and probably have a high, but finite number of charge cycles.

Cost might end up being the same. (Don't forget that you'll need some kind of equipment to recharge the lawn tractor battery after it runs down.) I would go for the FlashFish just because it's lighter and less hassle. My $.02.