I think an Export CofA would only be necessary if you're changing registers, which it sounds like you're not. If that's the case, and you're not a US citizen, you may also want to factor into the calculation the costs for setting up and maintaining registration in a US trust.
Other than that, the only other additional costs that spring to mind are:
- Ferry flight insurance
- FCC radio station license & RT operator permit (negligible cost, IIRC)
- Depending on the type, ferry tanks and potentially other pre-ferry preparations (GPS database subscriptions?).
- It may also be worth considering getting an annual done in the US before bringing it over. Also any installation changes that you want or need (8.33kHz radios? WAAS GPS?) will probably be significantly cheaper in the US than Europe.