On the modern trains, everything is upgraded from the interior passenger space, access, control systems and use of modern parts giving a more realistic look + playability on the new sets.
All the below commentary aside, cheapest you can get fully working ones with box on eBay etc is typically c£80-120+. I know some people don't like to deal on eBay (no seller fees now) and so cheaper local marketplace pricing isn't uncommon. Assuming this is all working with genuine Lego parts than £50ish is a bargain so anything else is a moot point really. You can pay that just for the engine carriage.
Depends on the 7897 set condition, is it boxed as new, is it well played with or a collectors set (motor and gears do wear)? Lego is pretty long lasting if looked after, even the electronics keep going as the early systems were basically so simple nothing to go wrong ie switches, connectors and wire.
Downside is, if you do need to replace any of the main parts you can end up spending as much as the set on this era of trains as they are long out of production and moving rapidly towards the vintage category.
As an example of electronic Lego set longevity, amongst all my original classic sets (castle, pirates, space) I still have my original Space 6990 Monorail. It still runs ok although obvs getting on for 40 years old. The minifigures are all still good and parts still look ok albeit a bit of yellowing on some.