Not every Mini dealer will do it.If I'd kept my RCZ I was planning on having a walnut blast done. Apparently the local MINI dealer did a decent job of it. I think at the time I was quoted around £300/£350.
Our local Peugeot dealer, who has subsequently lost the franchise was rather useless. Ok for ordering parts, 99% of the time.
I approached 2 official MINI/BMW dealers in Slovenia, both refused walnut blasting on my RCZ - they said they have no idea if it's possible to do it without taking out the entire engine.