Special offers are based on 

Time

the more time you spend without going buying the higher will be your offer. These time used to circulate at 1 month, e.g you buy at 80% of 45$, the price you will get next time will be arrounf 50-60% of 90$ then 60% of 45% ,then after 1 month it drop back to 80% of 45$ again. However it has been cut down to 3 weeks. That your percentage will regenerate after three weeks these is because we get offers on Tuesdays sometimes in addition to Friday

Geographical location

These have been tonned down. They fixed these in May after I exposed them for it. Now they have divided offers types based solely on quadrants. There are 5 quads( they could be more). USA east and west, central Europe and Africa, UK and East Europe, Australia and other. I have heard that there is an independent one for Ukraine and another for South America. The items that these quads receive are the same, eg you get 9$ offer but I get 19$ but we all receive the same types of units. The is actually what Lord Mark is asking, Why aren't those offer the same.

Player type, e.g purchase history

We are not all in the same scale, even if we are all in SA, We will interact differently with the black market. A logarithm is used to record what you buy most, and these is used to factor special offers given to you. So if you like Thieves then you're likely to buy an offer with thieves, and thieves you'll get. The algorithm is also applied on global scale. That is why most offers have more offensive units, compared to defensive units. VIP also recieve better offers. 

Knowing these three basic factors you can actually use them to your advantage. For example when I fell like spending a lot( over 100$) I use a VPN and buy in Viet Dongs and not in rands.