Johnson is currently at best an incremental improvement on Arbeloa, but is two years younger and has more potential upside. He's a better fit for the league as well, a better athlete.
If we get 10 million for Arbeloa, not only will it, along with Johnson set a new inflationary standard from this point on for the new cost of a decent fullback, it'll still mean we have to see the improvement on the pitch to justify the fee.
There is absolutely no doubt we are overpaying though.
Oh, and whether there was a rule or not, I've no problem with us trying to have some English core, and paying more for it, I've just never been wowed by Johnson, but am looking forward to seeing him with an open mind.