It is mostly dependent on what you buy. The developer discount is generally an additional $100 off macbooks for example, plus 20% off upgrades like RAM and hard drives (though you should buy your own RAM in any case, as Apple likes to rip off customers when it comes to memory).
That said, the discounts on the macbook pros are pretty good. IIRC, the base mbp goes from $1999 (regular price) to $1599 ($400 savings). Considering you put in $99, that means you come out with an extra $100 saved.
The trick to it really involves buying the most expensive machine you can, as this will save you the most and minimize the initial $99 cost.
Check out the store and see for yourself. If all you want is a macbook, don't waste your money and the once in a lifetime discount on it. 🙂