As Alisa pointed out there is often a 1 time activation fee.  This fee is charged when you first start a membership but as long as you maintain that membership it isn't charged again.  The activation fee is done to cover the cost of processing the membership and is always over & above the membership cost itself - and often hidden in fine print.

DAZ advertises the PC at $7.95 a month, however the actual cost for the first month is $29.95 with their "start up" or activation fee added to it. 

I wanted to also address the comment that things rotate in and out of Prime.  That's not completely true.  Prime items stay in Prime as long as the product is in the store (there is at least 1 vendor that I've noticed pulls hers from the store after awhile) only prime Classics rotate in & then out and they are "bonus" items from a Creator's older catalog that they offer for a special discount to Prime members.

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I do not feel obliged to believe that the same God who has endowed us with sense, reason, and intellect has intended us to forgo their use.
-Galileo