Originally I needed to add some crystals to my total (the amount I had would have bought 2 expansions at 75% off) however the offer was only 30% so I spent just over $2.00 to buy 2 expansions. Before spending just over $2 I always had 75% off and to achieve things in my game play was easier. Ever since purchasing crystals I have only received offers of 30% off for the expansions (3 to date). Regardless of having a nice supply of crystals or a few (after spending them and then rejenerating within game play) it has made no difference to the percentage nor did the level as from 1 expansion time span to another the level was only up by 2.
I understand it is a business however fairness would be appreciated; furthermore I would not recommend to anyone to spend money on the game whereas if there were benefits and profitable insentives to spending up then I would be keen to pass on the value/s of spending real money.
IMO it's about being stupid enough to spend money in the first instance leading them to think "Gotcha!"
PS: Good idea you have.

ERROR: % other way around..game offfered 30% not usual 75%

Ditto! This happened to me on the previous land discount. I knew it was approaching so I decided (first & last time) to spend $2+ to purchase crystals which enabled me to open a new area. I was so excited and planned out the area in my mind for the expansion however; it didn't offer 30% per usual but instead 75%....hhhhmmmm (again not enough crystals), logic told me that I was being dupped ; so I wrote to support and my reply was as yours-----'offers are made randomly and that I was just unlucky'. Awaiting this next expansion offer to see what % is offered.