When buying bots, you have different subscription options:
The extended periods (90 days, 180 days and 365 days) are progressively cheaper, in a "bulk buy" sense. Please note that only one count will be registered for the Bot Master achievement (and one for Little Santa, if you are buying for a group that you do not own), regardless of duration, but you get the benefit of the discount.
Choose this option for best value.
Alternatively, you can choose to buy multiple subscriptions of the same bot (of any duration) - even for the same group. For example, 6 x 30-day subscriptions rather than 1 x 180 day subscription. This way, although you won't get a discount, you will get all the benefits of bigger, better achievements and more rep!
Choose this option for maximum achievement and reputation benefit.