I must have a lot to either better understand or know better where to buy. From a small shop in Lotus (not DTAC) I just bought a 12-month DTAC sim for 2,500+ Baht (ie, 2.5 times what you paid).
I use it in a router (which I carry with me to use with my 4 pc's in 4 different Thai cities). It does have a larger monthly limit, I believe 100GB/mo.
I can also call with it if I put the sim in my mobile, but I don't need or bother with that feature since I have a postpaid ais mobile sim (approx 270Baht/mo & I seldom use mobile now).