On the ones I have been on (never a JEJames one) the discount has varied between 10% and 15%. They usually come with some sort of restriction, like an expiry date.
You may also be able to use them elsewhere to get an even better deal. For example, I used a Rutland Cycles 15% off (use by....) with JE James after they had already offered an at cost TEC pack.
By the way, I used JE James at Sheffield and their after sales service is excellent.