There was a guy from HM Passport Office on TV on Friday, who said that:-

1) The new blue passport will not be introduced until the current contract for supply expires in October 2019.

2) The only cost will be the cost of the new contract, GBP 400,000-ish over the life of the contract, which would have occurred anyway as a part of the renewal of the contract, even if the passport was not being changed.

3) people's existing EU passports will continue to be valid until the expiry date shown in the passport - it will not be necessary to renew your passport in Oct 2019, although some people may choose to do so, at the standard charge of 72 GBP for a new document.

So...it appears that there is no cost directly attributable to the change from the burgundy EU colour to the dark blue U.K. colour.

The only question the man failed to answer (I'd guess because he doesn't yet know) was concerning which queue holders of the burgundy EU passport should join at airports and ports after we leave the EU in March 2019 - 'EU Citizens' or 'All Other Nationalities'.