Have you ever explored Vodaphone unlocking it if you tell them you need to use it abroad on holiday.I thought I heard once that might be possible.Many countries need a local sim card as your provider does not always have an agreement with a local phone company. Just a thought.
Apart from that, I don't think it will be cheaper in Turkey to unlock as my Nokia wasn't and there are not a lot of HTCs around.
Hey get the HTC Sensation,newer and better LOL