Hi Jakuson,
Very glad to see reefers, like u, who are interested in rare fish like this.
The cook-island purple-head variant helfrichi (common name) is very rare and has never been brought into singapore.
So far those available has been retailing at pricing upwards of $1000 with majority going to Japan and a few to HK and Taiwan.
A few has went to the US market back in 2011 and the picture u posted belong to LiveAquaria and it sold for USD 600+.
Back in 2010 to 2011 the purple head helfrichi was targeted for collection together with another even more expensive fish, the privitera anthias ($1300 to $1500) in cook island at depth beyond 200 feet. They are only found deep, though not as deep as the peppermint angel and narcosis angel.
This purple head variant helfrichi is found twice deeper than your usual helfrichi and as such is rarely collected and command a high price.
Another interesting thing to note about this fish is it is also much bigger than the normal helfrichi, they are typically 3" in length which means it is almost twice as big as your normal helfrichi!
So this purple-head helfrichi if available will be a very strong and showy firefish to have and well-worth the money spent.
After 2010 and 2011 these purple-heads have not been collected again. And in 2012 the fish was practically absent from the market except for one being made available to Japan BBOX some time in the second semester.
I have been trying hard to look for this fish in 2012 but to no avail.
And like you i hope to see this fish in singapore one day and hopefully i can be the lucky person to own it.