Your not far off, but let's split hairs and say 9 years ago
It was released in 2006 as a cask strength single cask (53.7%) which was distilled in 1989 so a 17yo. Originally priced @ €90 from a cask which yielded circa 300 bottles.
Bushmills has released some wonderful single casks in the past but this has all dried up along with any enhancement of the brand.
Unfortunately Bushmills has suffered from a major image crisis since it left the IDL folder and even then it was not pushed too hard. They have come out with some wonderful single casks which have simply past the world by fairly un-noticed. Therefore it is hard to guage value.
I'm sure there are a few Bushmills collectors that would like to have this but at what cost is the unknown.
However if selling I would ask for minimum €100-120 but as for a bigger value I could not guess.
Bottles such as these should easily command double the original price but unfortunately that is not the case a lot of the time and is very hit and miss.