I believe toms hardware has a few numbers on SSD/HDD speeds. I would say a good estimate of raid 0 speeds would be 1.6-1.8x single drive speeds. will not be full 2x since the raid controller processing takes time.
Figure the cost of the drives you want to use, with the individual speed results of said drives... then factor in the prices you research.. that will give you your answer.
Next keep in mind that single ssd or raid0 both offer no backup options. If a drive fails you are SOL.