This is not an easy question to answer because there are a lot of factors that a doctor will need to to consider before starting or stopping Roaccutane. The best person to ask would be the doctor who has been treating you and following you up because he or she knows your condition best.
In my opinion, it is not ideal to stop it immediately. You may want to continue on a low dose for a period of time as a maintenance. This is to avoid a rebound acne breakout, which can happen if one stops taking Roaccutane suddenly.
In summary,
1) Give your doctor/clinic a call to arrange an appointment to see your doctor
2) Explain your concern - medication is expensive
3) Let your doctor assess your condition and advise you further. There may be alternatives that your doctor can recommend