The shop diagonally opposite to the Bangkok bank in the town centre also has dashcams in the window.
A taxi driver told me that all these devices break in a year or so, so there's no point in buying an expensive one.
I have a Garmin NuviCam (all in one GPS with HD DashCam price was 15k THB) and I now have it over 1.5 years and still works perfectly.
I have no idea about No Brand Name stuff as I will not buy them, but the Transcend DrivePro are worth every penny IMHO, but I opted for an all-in (GPS/DashCam) for the Front Window, and a Garmin GDR-33 for the back Window. But If that GDR-33 Fails (it's still going strong 2 years on) I will go for the DrivePro series.
One more advice. if you have a chance, test the Dash Cam you wanna buy (or look up video's on youtube) how it performs at night. For example, can you clearly identify License Plates Numbers ?
The first DashCam I bought (cheap Chinese s...t) was so/so during the day, but at night you couldn't even identify a license plate number when the other car was only a couple of meters away