a card from MB for the GLE500e costed £650
Assuming you are referring to Garmin Map Pilot, I think you'll find that there are two versions of the SD Card.
The 'first time' use SD-Card is intended for cars that never had the Garmin Map Pilot SD-Card installed before, i.e. this is the first time that this option is being used in the car. This SD-Card typically costs in excess of £500.
The 'update' SD-Card is intended for use in cars that already have an older version of the Garmin Map Pilot SD-Card installed, as an update that replaces the old SD-Card. This SD-Card typically costs around £100 for the genuine item (copies can be purchased off eBay for less).
However, apart for the labels on the SD-Cards and the MB Part Numbers, both SD-Cards - 'first use' and 'update' - are... identical.
No info regarding the SD-Card is stored in the HU, and so the HU does not know if the car had the Garmin Map Pilot software before or not (it works the other way around, i.e. the SD-Card retains information regarding the HU, and it is 'locked' to the HU after first use).
If you insist at the parts counter at the dealer that you need the 'update' SD-Card rather than the 'first use' one, they might eventually sell it to you.