Mountain Bike Disc brake pads price vs performance I know the differences between organic and sintered pads. That bit is (fairly) easy.  

Where I come unstuck is finding out what is the difference between pads at different price points. Does anyone know what the difference between a set of $10 set of disc brake pads and a $40 set is likely to be.
TL:DR bullet point version at the end.

Since you do not specify the pads at those price levels, I chose to interpret the question as what are the potential differences between brake pads.

You said you know the difference between resin and sintered (there also exist semi metallic pads as a kind of middle ground), well resin and sintered is a huge difference (see end of answer for more info), you will not experience a difference anywhere near that magnitude between pads within each compound.

The next "biggest" difference is heat dissipation (which is actually also a difference between sintered and resin) I'm sure there are some other options out there, but I'll stick to the one I know:

Shimano Ice-Tec finned pads give better heat dissipation, this reduces fade and improves pad life (both in theory and in my own experience).

Lastly there are some minor differences like titanium backplates or slightly different compounds, never been able to tell the difference between different resin compounds or different sintered compounds, except once when I had a pad emergency and got some horrendously fast wearing and crap braking resin pads from an LBS, can't remember what they were, but they were the most expensive pads I had had at the time (pre ice-tec days).

The above are the real differences, beyond that, it's just brand, marketing and profit margins (not that I could tell the difference between normal and Ti backplate sintered shimanos, so backplates are crap unless proven to reduce temperatures... although the backplate would make a difference if you wore through the pad and used the backplate...)

Oh there are also people like Mr.Magura on mtbr forums who have made carbon discs and pads, while his creation is impressive, there have been some attempts at commercial versions but all have been total crap as far as I am aware, but it's worth mentioning that pads for those discs no doubt have a lot of differences to standard pads.

Resin vs. Sintered, 

Sintered have:


More bite (and less modulation).
More stopping power.
The ability to stop you effectively in the wet.
Less fade.
Slower wear on the pad (but faster wear on the discs)
Compatibility issues with some (usually cheap) disks.
A Reputation for being loud (but I don't find this to be the case, IME there's very little difference, unless you haven't bedded in properly, or haven't set everything up correctly.)


Resin has:


Better Modulation (less bite)
Allegedly make less noise.


TL;DR Version:


Heat dissipation via fins = less fade. (important to some people, but
not most)
Minor compound differences, back plate material. (irrelevant to most
people)
Marketing bullshit and profit margins. (irrelevant to most people)
Resin pads (+modulation +quiet? +wear on pad) 
Sintered pads (+power +wet performance +wear on disc +less fade)
Semi metal pads (between the other two)