if you thinking about FP64 maybe you should check out used 7870 they have 0,7 tflops in fp64 and 3tflops in fp32, you can get them for around 100$ or if you want also better performance in fp32 you can check R9 290X they have 5,6tflops in fp32 and 0,7tflops in fp64 for about 150-200$.