Well, I was just able to price a similar trip on Expedia using multi-city (SFO-CPH, TXL-CDG, CDG-SFO). It's in F class (and J class on the euro legs) but $15k+. Probably would have been cheaper to book TXL-CDG separately, even in J! Fare basis code is FN1H06F1, and it's $800 per stopover. (Also $500 cancel fee, $450 change fee). Of course this is relatively close-in, I would gather longer A/P would result in a lower fare.

ETA: Excluding the TXL-CDG leg prices at $878 less (stopover fee plus I guess some taxes). One-way TXL-CDG booked separately in business is pricing at $561. A good travel agent probably would have caught this and booked/priced that leg separately.