what should I pay a sales representative as commission to keep him motivated to sell?

Why don't you consider paying him a basic salary of approx £100 per week, so he doesn't starve if he has a bad month. Otherwise if he has a bad month with no pay he will leave. You don't give any figures, so it's difficult to quote. Do your research, consider what others are paying. In retail it can be around 3 per cent commission and in car dealerships the sales staff get about £50 for each car they sell (inc. a low basic salary of about £600 per month). Remember, good sales reps can earn a fortune £50,000plus a year simply because they demand large commissions, the rest are a nightmare because they leave as they can't earn enough to live on. If you have a team of sales reps it's sometimes a good idea to reward ALL the staff a small amount for EACH sale, that way they all work to the best of their abilities for the good of your company. Good luck.