This could have been posted under General Europe or Transportation, but since my trip will be entirely in Germany, I'm posting it under Germany.
I'm currently planning a 21 day trip for 2 to Germany. I will start at FRA and go to a small town on the Main river in Bavaria, from there to Fischen, in the Oberallgäu region in Bavaria, then to the Mittel Rhein (St. Goar), then back to FRA. I have planned 16 days with rail travel, four of which are major changes in venue and 12 off which are planned side trips from my venues.
I already have one day with an Advance Purchase Savings Fare ticket already purchased. Most of the rest of my travel will be using Bayern-Tickets, local Verkehrverbund Tageskarten, and full fare regional tickets. My total expected ticket expenses, this way, is 435,40€, about $465.
To compare these expenses with a rail pass, I listed all 16 days of travel on a spread sheet and sorted them from most to least expensive. I then added a column for the cumulative expense, converted to USD at today's rate. Beside the cumulative expense, I showed the cost of a German Rail Twin Pass, including the then-current 20% discount for 7, 10, and 15 day passes. My three most expensive days of rail travel total $146 (137 euro) using tickets. A three day GRP Twin would cost $338. At no point (number of days) is the cost of a rail pass less than the expected cost of tickets. The closest the rail pass came to break even with point-point prices was a seven days. ($386 for the rail pass vs $274 for cumulative point-point tickets). After 7 days, the next 8 days would have cost $180 for point-point tickets vs $300 more for a 15 day rail pass, so a 15 day pass would not be economical.
So, the rail pass option would cost $686 for a 15 day pass vs $453 for tickets. If I only got a 7 day rail pass and bought P-P tickets from there, it would still be $577 for the rail pass option vs $465 for tickets for my full 21 day trip. It should be noted that without the 20% off special, the rail pass would not be as competitive as it seems.
Finally, vs a car rental. The most-often-here-recommended car rental broker gave me a quote of 431€ for a compact car for 21 days. ViaMichelin estimates my cost of fuel for the 16 days of travel at 262€. Total cost of using a car, 693€, or $734, $270 or 57% more than point-point rail tickets.