No Idea about Ideos but all other phones cost about the same here in any mobile phone shop give or take a few hundred baht. HTC might be a little cheaper, Galaxy a little more expensive.
I think NZ and Thailand are bundled in Asia Pacific market and so have the same models and the same prices. Of course it varies from manufacturer to manufacturer but there's less difference in the region than between Asian and North American markets.
So buying in Thailand shouldn't be much different than the UK either. I am going to wait and see what I can find in LOS then.
We don't have nearly as many models as in the UK, but all our phones are unlocked. If you bring unlocked GSM phone from there it would work here, too.
Mobile phones are cheap here as it is, it made sense ten years ago when they were locked to carriers and were ridiculously expensive but I don't know why bother now.
The only reason I can think of to get an imported phone is some cutting age, top of the lines models that are not yet available here. That's why we had Hong Kong iPhones and iPads and Galaxy Tabs all over the place, starting with MBK.
Nexus S might fit into this category.