MSRP for the battery is $1000, perhaps there's an installation and firmware update charge? They're hard to get too...
There's lots of upgrades on the newer Vado's depending on which one you're looking at. Biggest difference is the 4.0 and 5.0 for 2020 and beyond are class 3 bikes, better drive train, better brakes. If you don't need the class 3 or much more powerful motor of the 5.0 then the battery upgrade might work fine for you.
If your current Vado has the older Blox display, rather than upgrading to the TCD-W and battery, a new bike might not be a bad idea. We've got one TCD-W left and I think I might be able to get a battery. TCD-W upgrade for parts alone is $160 I believe. The improved brakes on the 5.0 as well as 11 speed drivetrain and the motor make it the best deal around, my $.02