Why does the price for a prescription drug fluctuate? "In drug price hearing, Congress tries to answer some basic questions" - dateline June 2017 CNBC.

I order a brand name prescription that has no generic, that is still under patent protection, that is not an orphan drug from a large mail order pharmacy. I have been getting this same prescription for six years. The mail order firm buys direct from the manufaturer, adding their overhead and profit to the price and then is paid by (1) my direct payment and (2) payment from my drug prescription insurer.

Over the past two years (in particular) the sum of payments (from 1 and 2 above) to the mail order pharmacy has varied substantially, and inconsistently (eg. increases and decreases) from a low of \$1400 to a high of \$2600 for the same 90 day order. 

To be more specific in my question, why would a manufacturer price for a drug fluctuate as I describe. 

One area That I've not been able to research is if Insurances companies negotiate pricing with the manufacturer -- but if that is the significant factor -- it suggests that the negotiated pricing only holds for 90 days or so.

Any insight would be helpful.

If their is a better place to ask this question, please suggest.
Prescription drug prices, like those of most other retail products, fail to satisfy the law of one price routinely. The most obvious observation is that prices differ across pharmacies at a given point in time (http://www.nber.org/papers/w8548.pdf) but just like supermarkets give discounts, so do pharmacies. If you always buy from the same shop, you are bound to end up paying different amounts. 

The reasons for this are manifold. Different consumers have different abilities or willingness to store and buy large chunks when it is cheap and stochastic prices allows for price discrimination (see Sobel 84 for an early reference). The idea is simple. Myopic consumers arrive in each period and buy if $p&lt;V$, then leave the market. Otherwise, they die. Strategic consumers have a valuation of $v&lt;V$ but can wait if the price is above $v$. Firms compete in prices. In equilibrium, firms charge often $p=V$ to extract rents from myopic consumers but every now and then they offer $p&lt;v$ to attract all strategic consumers looking for prices.

Competition between stores may also result in random prices in a standard consumer search framework. Finally, retail prices may change because manufacturers prices are also random as a way to discipline pharmacies by encouraging consumers to shop.