To put some perspective, I just followed a TI advert that showed up here on the EEVblog banner about their A57 Sitara processor.
The eval kit (which has a ton of stuff, granted) is $599
http://www.ti.com/tool/tmdxevm5728This is for a dual core ARM, but it does have a quad core PRU, extra cortex-m4 for floating point,DSP, etc, so yeah it's probably worth it. But the chip itself costs $75 in 1K quantities.
Secondly, you can also get a Jetson TX1 with eight cores big.LITTLE 4 of them are the ones offered here ( A57) for the high performance, the others are A53 for low power. The eval kit with the platform is about $500 as well, but it does have an Nvidia Maxwell based GPU with 256 cores fully CUDA capable and does the full OpenGL 4.4 (and 4.5 as well) which spanks the Mali 400 dual core.
Just the TX1 module without the base platform goes for $299 in 1K purchases (so you can't buy just one).
Granted the TX1 offers a ton of other goodies, SATA, PCIE, USB3.0, etc. with the CPUs running at 1.9GHz memory (DDR4) at 1.6GHz and the GPU at least at 1GHz.
Anyways, I didn't look too deep into other quad cores A57 based dev kits, but $19? it's a no brainer. True I could get a consumer product that can be hacked, but it will always have some hidden features that are not documented, plus no GPIOs unless you can find them.
For example the Nvidia Shield TV (TX1) was on sale for Cyber Monday for $150, but it's not as flexible as the pricier Jetson TX1 and you have to live with the design at hand and power consumptions of 10w , not much but 2.5w -3.5w of this $20 kit makes it more interesting to me.