Well, some from the books and some from real-world applications.
I couldn't remember where the rule was about being able to chose the order in which bonuses/multipliers were applied.
I was gonna try to tie a correlation between that and the Coupon Policy for some stores / disclaimer on some store coupons "..discount applies after any applicable sale price".
I was under the impression that 10% Store Sale + 10% Store Coupon was this:
cost = [retail - (10% retail Store Sale)] - (10% retail Store Coupon) = 19% savings
...while the following was stackable for Store Coupons + Manufacturer's Coupons:
cost = retail - (10% retail Store Coupon) - (10% retail Manufacturer's Coupon) = 20% savings
I found it funny that non-gaming folks actually used the word "stackable".
This is probably one of the worst arguments I've made for D&D, but hey, at least I tried right?
The reason it doesn't inspire much confidence in me, is I can't find any Manufacturer's Coupons that are variable; they're usually a fixed amount like $1 off.
We can reverse engineer the percent savings, but I'm not actually buying that arguement myself.
In the case of a $1 off store coupon and a $1 off manufacturer's coupon used for a $10 item, they both reduce the cost by 10%, and together save the savy shopper 20%.
...yeah, I'm not buying it either.
The other arguement was the for order of effects. In the example in the PHB, there is a progression of events; step 1 large size then step 2 turn to stone. The same applies for applying a store coupon 'after' a store sale price is applied. I was thinking that the cost reducing feats might look different if they are seen as simultaneously applied.
Instead of: cost = [market - (market x 0.5)] - (market x 0.75) = market x 37.5%
I was thinking along the lines of: cost = market - (market x 0.5) - (market x 0.25) = market x 25%
The really sad thing is, when I actually did this math for the background of an NPC in my campaign years ago, I used the same math you did. I just multiplied all the modifiers together (0.5 x 0.25 x 0.25 x... etc). I had forgotten all about it.
I have a NPC hordificer that mass produced rudimentary intelligent effigies kinda like that proposed in the neighboring thread. I've followed snizor's Mechonomicon thread and participated in the Mr. Robot thread (I'm Ninja1 on 339). I totally forgot about the math involved.
Thanks for going easy on me and my addled mind.