Astromech Droid, R2-D2, Light Bluish Gray Dome

Comes in 11 sets, made of 4 parts. The lowest-priced set it comes in is 5002210 C-3PO and R2-D2 Buildable Watch with Toy (RRP £19.99).

Check the minifigures carefully when buying a used set: they're part of its grade.