I’m just starting to get all the components and start with the custom tonies.
Just ordered this combination, as already mentioned in multiple comments. Sorry, can only share 2 links as new user:
Clamp, currently 5,49€ from Amazon
UART-TTL USB Adapter, currently 6,49€ AZDelivery USB TTL Adapter 3.3 V or 5 V with CH340G and 20 cm Jumper Cable: Amazon.de: Electronics & Photo
ICODE SLIX-L RFID-Tag, currently 0,80€ / piece: Pack of 10 ICODE SLIX-L RFID Tag Stickers Compatible with Toniebox, NFC Chip, Self-Adhesive : Amazon.de: Stationery & Office Supplies
Inlay, 0,15€ / piece with 12.5mm hole from elecTricky
Coin-Boxes, 40mm, 0,29€ / piece from elecTricky
Magnets, 0,30€ / piece, 12,5mm x 2,5mm from elecTricky
So i get a total of 1,54€ / custom Tonie (+ some paper for decoration).
One-time costs: 94,46€ Total
Used ESP32 Toniebox for 47,00€ (Kleinanzeigen) + Clamp 5,49€ (Amazon) + TTL Adapter 6,49€ (Amazon) + used Raspberry Pi 3 B incl. Accessories 27,00€ (Kleinanzeigen) + Micro SD Card 64GB 8,48€ (Ebay)
All-in-All a pretty good deal! Thanks to all of you for sharing 
Hopefully all parts work together