John,

> My 1st copy of software is costed about US800 dollar. How should I charge for the following copy? It is my 1st time, therfore I also don't know to to mark the price. Do you think 10-20% discount is reasonable? It is too high or too low? I heard from somebody that, the following copies should be charged only about US100-200 dollar. Is it too low!!???

Why not offer additional copies to the customer at 20% discount, and see what happens?

If they agree, you've made some money.
If they disagree, you then are in negotiations and can find out what THEY think is reasonable. THen, you try to meet in the middle somewhere.

Number one rule in negotiations: Ask for more than you are willing to take.