The SSD pricing is sure dynamic. So it's nice that Tom's Hardware updates this regularly which updates the Anandtech Best SSDs for July 2014 as well as StorageReview.com which did a good comparison of the Evo 850 Pro, Evo 850, Evo 840 and the Sandisk Extreme Pro which showed the 1TB Evo 850 very good at synthetics and both the 256 and 1TB excellent at the real world.

• Evo 850 vs Evo 850 Pro. The Evo 850 like the GTX 970 (vs GTX980) is the best price/performance and also uses the V-NAND. You get nearly the Evo Pro performance but at price closer to the budget levels.

So here are the current recommendations:
Budget buys that are good for bulk storage and where highest performance isn't a huge issue and the Crucial MX100 is a good deal.

• Crucial MX100. 256GB. $119. • Crucial MX100. 512GB.$225.
• Evo 850 1TB. $500. This is interestingly is the best value in the whole list and a high performer as well. Have you're cake and eat it too. Performance matters but don't want to pay a super arm and the leg • Evo 850 256GB.$139.
• Evo 850 500GB. $270. • Evo 850 1TB.$500

Net net a good configuration might be a boot drive which is a Evo 850 256 and then as main storage in Evo 850 1TB. Of course if you want lots of desktop storage, then a single spindle 4TB drives but reliability is always an issue.
Then for mSATA, they recommend the Evo 840 in all its sizes.
They don't have an m.2 recommendation but looking at other reviews, it looks like for the more common m.2, there are three things to think about when mixing in pricing. The bigger issue is that the faster drives like Puget systems measure meant of the M6e and the XP941 run very (like 94C to 104C hot!).
Best budget. These use SATA controllers so are limited to SATA 6Gbps (500MBps) speeds and cost is the issue and I've used the Crucial M550 m.2 and it booted find on a ASUS H97I-Plus.
Best PCI Express x2. These are native systems that use the full 10GBps and the only one I know is the Plextor M6e but it is so much more expensive (typically 2x the regular SATA), so probably not worth it.
Best PCI Express x4. This the only system that supports the full bandwidth of 20GBps and use the very expensive Samsung XP941 which also runs very hot in the native slot, so you probably only want to use that if you have a full PCI Express.