Train Berlin to Frankfurt

There is no alternate. But you could look at the cost of airline tickets. What dates, and since there are a lot of trains, there is probably something cheaper, maybe not at the precise time you want or involving a connection.
And 85 EUR per person is not too bad. 29 EUR pp is about the best you could hope for.

As far as the reason, only a certain number of tickets are offered at the rock bottom prices. After they have been booked, only higher priced tickets are available. A full fare Flexpreis ticket is 123 EUR, so 85 EUR is still a discount.

The cheap tickets are indeed probably sold out for your date. I checked and even for the beginning of December there are tickets available from EUR 24, depending on date and hour.

"Is there an alternative rail company I could book with?"

You might snag a better price by using one of the long-distance bus services like Meinfernbus.
https://meinfernbus.de/en

What are your travel dates? For the first week of december I see prices starting at 19 E. Flixbus prices start at 22 E. for the fastest connection (6:30 against 4:15 for the train). Flights are from 65 E (Air Berlin).

The Berlin-Frankfurt train has been full or nearly full on occasions I've used it. DB probably has little or no incentive to lower the fares on a high demand route. I'd definitely get reservations, early.

From your previous post about being in Berlin for New Years Eve:
Tickets for the 7:34 ICE to Frankfurt are 31.90 EUR on Jan 1, 67.90 EUR on Jan 2, 24 EUR on Jan 3, 19 EUR on Jan 4, and 43.90 EUR on Jan 5. And that is just that particular train. Looks like you have lots of options.

Not helpful for you, sorry.... but for other people following this thread - we always book the train tickets the day they open, and we got our family of 4 (2 kids) for total 57 euro from Berlin to Frankfurt on Dec. 31.

I was a couple of hours late booking our tix this time and there was one leg of our trip, I can't remember which one, where some trains of the cheapest tickets were already gone.... those cheap tickets go fast!

So just word of warning for people gathering info.... watch the open date and if you know your dates, get your tix ASAP.

To OP, I hope you find a good solution!! :)

Kim

Just to add to the bad news, what Kim says is true for more and more trains in more and more countries (including, by the way, the US). Those who know about this, know their plans for sure (the discount tickets either restrict refunds and exchanges or eliminate them entirely), and can book early can get good prices; those who don't know or can't book early, have to pay a lot more.

The bus option is a good one in places where these compete with trains, for those with more time than money, or those who weren't able to book trains in advance (the bus prices these days can go up closer to travel, but not as sharply as trains do).

DB probably has little or no incentive to lower the fares on a high
demand route.

I don't think DB ever lowers fares on any route depending on demand. The full fare is based on a specified cost/km that is standard across the system, but there is a maximum cost per class for any trip across Germany. However, their discounted (Spar or Savings fare) tickets start at 29€ pp and go up as the number of tickets at each price level are sold out. Depending on demand, they might allot fewer tickets at the lowest levels or the least expensive ticket sell out almost immediately.

If you are stuck with a certain travel time (date and time), you're stuck with what they offer. If you can be a little flexible you can usually find less expensive fares. Early morning or late evening are less desirable and don't go up quite as quickly. Connections with one or more changes of trains can be less expensive. Try unchecking ICE from modes of travel. Connections using an IC rather than an ICE take longer but are less expensive to start with and discounted ticket are less likely to sell out. Lastly, try putting in an intermediate stop that's not on the normal ICE route. The trip might be a little roundabout and take longer, but you might find lower priced discounted fares available.

the discount tickets either restrict refunds and exchanges or
eliminate them entirely

Before the day of travel, Saving Fare tickets can always be refunded less a penalty of 17,50€/ticket. There is really no such thing as an exchange; you just refund the ticket price less the penalty and apply that to the going price at the time. On the day of travel, there are no cash refunds, but you can apply the price you paid, minus the penalty, to a full fare ticket on the same route as your original ticket.