Maximizing Bonus...
As I see it, the bonus system provides the greatest benefit to someone new to DCS who intends on filling their new virtual hanger with a lot of aircraft, terrains, and campaigns. The key is try to lay the foundation for a 9-product order to get the 60% (!) TRIPLE bonus, vs the 20% for single purchases. Campaigns are the cheapest products and there are a lot of them. At $10 a crack, you'd spend $90 to maximize the bonus and get $54 towards a future purchase. However, there are a couple of caveats.
First off, to purchase a campaign you must already own the associated aircraft module. In many instances, you must also own a specific terrain module. Secondly, outside of bonus-related specials, bonus $'s can only be applied to a maximum of 30% of the cost of a future purchase. As someone who owned only one DCS product (snagged Hornet for 25% off +$16 bonus), I decided to focus on western 4th-gen planes plus associated campaigns.
I made a limited purchase of aircraft to fulfill the campaign-purchase-prerequisites; i.e. A-10C and F-15. The cost was $75 which returned an $18.75 bonus and opened up 7(!) campaigns for the next purchase. Those 7 campaigns (6 A-10C, 1 F-15) cost $70 and returned a 50% bonus ($35). If I'd added Nevada to the first post-Hornet buy, I could have added another few campaigns and reached the 60% max. bonus. ;-)
So now Saint has invested $205 in 3 jets and 7 campaigns...and has a $70 bonus credit. As it can't be applied to sale items and can only normally knock 30% off a new purchase, I'll wait and hope another bonus-promotion comes along. Worst case, I'll save 30% on Harrier, Persian Gulf, etc when I have another moment of DCS weakness. :-)
Based on how much I've spent in past decades on flight sims and the current depth and quality of DCS, I'm content that the cost has been (and will be) more than fair...