The LED's I am using are rated for 30 Ma. I run them at about 25 Ma.
As far as the capacitor cost. All of mine are just used take offs from old TV and power supply boards. Same with the diodes, resistors and electrolytic capacitors!
Many of my LED's are from old user control panels from old office equipment. A large commercial copier control board from the 90's can have over 100 LED's and most of them are the same!
We have 120 volt mains. Running 40 LED's with a average forward drop of 3.3 volts at 25 ma means the capacitor only sees about a 35 volt peak across it. The average RMS will test as even lower.
So if you factor you are not running the full line voltage across them the actual working voltage across the capacitor is not going to be as high and thus doesn't need the normal high voltage rating associated with mains powered devices.
It may sound odd at first but the numbers do in fact work! My first green LED set up has a 100 volt non polar poly capacitor and its seen years of continuous run time without problems and thats proof enough for me that it works without reasonable concern.
I know my factory made LED cabinet lights are never going to make the same run time without fading right out of existence.
As far as power cost I am normally not one to nit pick the power ants but realistically if you change from a 2 watt loss using resistors to a .02 watt loss using good poly capacitors or similar in the 100K hour rated life span of the components thats still a few tens of dollars worth of electricity.
Just my theory.