What are the average monthly cost for each subscription?
What is the minimum/maximum a single subscription cost in a month?
Which month is usually does the minimum/maximum occurred?
What are the average monthly cost for each subscription?
What is the minimum/maximum a single subscription cost in a month?
Which month is usually does the minimum/maximum occurred?
It varies wildly.
Some subscriptions are shipped every month. [Adventure Path]Some subscriptions ship 3-4 times a year [Rulebooks]
Some ship 8-10 times a year. [Maps]
Some ship 4-6 times a year. [Pawns]
If you want to get a feel for the numbers you are asking for, scroll to the bottom of the page and click "Release Schedule". The column on the right will tell you which subscription line includes that item.
There are nine Pathfinder subscriptions and 5 Starfinder subscriptions. If you use the top menu, select Store>Subscriptions and you will be able to examine upcoming and currently available items for each line.
If you want to make precise calculations, search the Customer Service Forum for "New Releases And Subscription Shipping".
If you sort results by "newest first", you can scroll through the posts and pick out the last 12 or 18 months, write down what actually shipped each month for each of the 9 Pathfinder lines, visit the product page for each item, write down the price, and then do the arithmetic.
You will also need to calculate the discount that is available if you subscribe to 4 or more product lines.