Every set will be different, due to how bad they are to prep/sticker removal etc etc, but if any stickers are removed, I'd say half your price wouldn't be far away. And that's for '3' boxes!
As stated, he's not a business, just happy to help out if someone covers his time.
I did mine about a year ago with Matt black 3M wrap. Materials were £75 ish and about 15 hours of patience to do the 3 boxes. There's no way I'd do it for £100 for someone. Still looks good as new a year on though