The Starbucks here are really for Hiso or wanna be Hiso Thai's. Its a super cool American coffee shop with very expensive prices. Thai's will go there, spend 180 baht on a coffee then take a selfie of either themselves and the cup next to them or just the cup itself then post it on Facebook and/or Instagram, Tumblr, Snapchat, Tango, Whats App, Line etc... so they can show all their friends how Hiso they are for spending so much money on a cup of coffee!
Or at least this is how my Thai girlfriend explains it to me when I ask why Thai's go there and pay that much for a cup of coffee when there are so many other choices.
To me that just sounds like your gf is taking an opportunity to have a pop at HiSos. My girlfriend, who certainly isn't HiSo likes Starbucks, as do most of her friends. They don't drink it all the time because, as you say, it *is* expensive.
When I'm in there with her I see very few people taking selfies and posting them on social media. While I'm sure the practice does exist I highly doubt it's Starbucks' core market. What I do see is a lot of working-class Thais on their laptops, or reading magazines, and a sizable chuck of holiday makers.
As to why would a Thai go there and pay over the odds when there's so many other choices around, you could surely ask the same question of any nationality? I know it's almost as trendy to dislike the place as it is to be taking selfies there.... but what most people who dislike Starbucks seem to forget is that they would never have gotten so popular if they didn't make a product that a lot of people actually like
. And that, I think you'll find, is the reason that the majority of Starbucks customers go there - they like
Personally I don't drink coffee, so it doesn't make much of a difference to me.