What's the powertrain warranty on the last pure ICE S-Class you bought? I'd imagaine it's a similar scenario - just because that's the guarantee, doesn't mean that's all you get.
That seems on the high side...
From elsewhere on the web...
Mercedes hybrid battery replacement cost
Mercedes C350e battery replacement cost: £7,000 - 15,000
One of the biggest names in luxury vehicles, Mercedes offers many of their cars with a hybrid powertrain. Combining the complexity of an electrical drive system with the Mercedes' reputation for expensive parts may be a recipe for disaster for your wallet.
In addition to the parts being more costly than other cars, the labour charges at Mercedes dealerships are among the most expensive too.
Hybrid Battery Replacement Costs
Toyota
Prius: £1,500 - £6,000
Yaris Hybrid: £3,000 - £5,000
C-HR : £3,000 - £5,000
BMW
330e Hybrid: £4,000 - £9,000
530e Hybrid: £4,000 - £9,000
X5 Hybrid: £4,000 - £11,000
Mercedes
C-class Hybrid: £7,000 - £15,000
GLA PHEV: £5,000 - £12,000
E-class Hybrid: £7,500 - £15,000
MINI
Countryman Cooper: E £5,000 - £11,000
Cooper: E £5,000 - £11,000
Mitsubishi
Outlander PHEV: £4,000 - £7,000
Ford
Focus: 1.0 mHEV: £4,000 - £6,000
Fiesta: mHEV: £4,000 - £6,000
Puma: ST-Line Hybrid: £5,000 - £7,000
Also, Lexus batteries fit well within this range too...
Best quality new hybrid vehicle batteries for many vehicle types at great prices and long warranties
www.hybridrepairservice.co.uk