Sweet spot in the Shunyata power cords line up ?

I myself would suggest not going lower than the Cobra. I tried the VTX cords and they were decent, but while it can give you the Shunyata sound, it will be less obvious how good the line is that if you go with a Cobra Zi-Tron, or Python Zi-Tron, both above your price level (well, new anyway. Used Cobras go for around $750).
Whoa! I take that back. I just went to Shunyata's site, and they now have a Viper Zi-Tron. I haven't heard it, so I can't say what it sounds like, but Music Direct - where I buy many of my audio components - does have a money-free 30 day guarantee (which is why I buy from them!), so if you don't like it, you can call and just say that it doesn't elevate your system to the level you want it to be at.
This must be a pretty recent addition to the lineup (maybe past 2 months or so). I have all Shunyata power cords and I peruse the site once every couple of months, so it can't have been out that long. Give it a try. You have nothing to lose!

I don't know what "Gbmcleod" is thinking, but I hear great differences with the lower priced Shunyata cords. I use 3 Copperhead cords (400.00 each) and 5 Diamondback platinum (300.00) cords in my system in conjunction with a Shunyata Hydra 2 and a Hydra 4 power conditioner. Everything made a big difference, with the noise floor lowered considerably.

If you want a lot of bang for the buck, the Shunyata Venom 3 (newer ones) have been rated ridiculously good by TAS. It is one of their budget components of the year. Personally, I wouldn't spend more than 500.00 on a power cord.

11-04-13: Stereo5
I don't know what "Gbmcleod" is thinking, but I hear great differences with the lower priced Shunyata cords.

Personally, I wouldn't spend more than 500.00 on a power cord.

It sounds as if you do not have the experience of Gbmcleod, which probably explains why you wouldn't understand his thinking. I was never very impressed with Shunyata's less expensive offerings like the Copperhead or Diamondback either. I agree with Gbmcleod's reco to stay at or above the Cobra level in the Shunyata line. Pani, if you want to stay under $500 per cord, you would probably be better off using something other than Shunyata. Though if you are patient you may find a Cobra for $500.

Guys, how about the Python Helix which appears very often audiogon for about $500 ?

Secondly, is there any special recommendation on which cord to use on low powered devices like phonostage and what to use on amps ?

The sweet spot is the store owners pocket. That's where all your money is. If you want to change the sweet spot, you really need to listen before you buy anything.

To really get to know a power cord you have to have it in your system. The older Python Helix does fit your $500 budget, but from everything i've read the Anaconda is much closer to the King Cobra. Some even like the Anaconda Helix over the Cobra. I know the Anaconda can still sell for close to $1k but it could be worth the extra money to hear one of SR's best. If $500 is your strict limit consider the Neutral Cable-Mantra 2($400 1.5m). Everything i've read leads me to believe the claims of Nordost dynamics and speed with the musicality and texture of PAD. Check TMH Audio if this pc might interest you.

I could also get the Black Mamba CX HC for about $500. I want to keep it under $500 because that way I can plan to upgrade the power cords on all my equipments. By any chance does Shunyata cords slow down the tempo of the music, like many other "fat" cables do ?