Hi, as far as I know the low price packages didn't changed dramatically over the last years (for small amout of purchase), and they are decided mainly by automatic computer calculations. I spend little on this game, and track the packages for years, so you can say I am an expert on low price packages :).
Preliminary condition to get a good low price package is to wait a month without buying. Sooner or later you will get 2 good packages.
package 1 - 10$ for 20k drachmas + units. With this and 50k drachmas + 20k free drachmas, bottom line is 25$ for 70k drachmas + hundreds of phalanxes and other rewards.
package 2 - ratio of 1:3 for $ to drachmas, without units. This means for 20$ you get 70k drachmas.
Notice if you buy to buy all at the same time. Waiting for the next day the prices will increase a lot.
When you spend drachmas, only during the 35% refund event or during a bigger discount daily event.
And persian positions, I reccomended in another post not to play it too much, as you loose a lot of army/real life money.