I just do what I do with Steam summer/winter sales, never buy anything prior to the last day of a sale unless it is on a daily deal/flash sale, and even then I won't buy it unless it's at or near the lowest price I expect it to reach in the next 6-12 months. Practically (and based on previous sale patterns), that means for ED I don't buy unless it's at least 60% off.
Whilst I have preordered the Mig and will do so for the Hawk, they are both dream aircraft for me, I won't spend more than I absolutely need to in order to get my hands on them. It is simple financial prudence (rather than availability of cash, at least for me) to adjust your shopping habits according to the established sales patterns, whatever the product. Between DCS modules I already own, the ones I will get from the Kickstarter, and my current P3D hangar, I'm not going to run out of things to fly any time soon.