Depending on the book, in my experience, you don’t often get the luxury of increasing the extent for other languages. Most of the books I do already have the rights sold to other territories before they are even printed in English, so you need to allow for a text (5th), overprinting black and all text appears on the fifth plate. That way the publisher can sell out the co-ed rights. Makes producing content-rich trade titles more cost-effective for the publisher. The 4-closet doesn’t change, just the 5th black.
As to design cost. That depends on a whole lot of factors. The publisher, your experience, your ability, the type of book, the complexity of the book, where you / the publisher are based, etc, etc. For an illustrated trade title, including photo retouching and balancing, etc, Smurf’s 4p/ph is not far off the mark.
As to timescales, the publisher should usually issue a draft schedule ahead of content delivery for you to agree to. For a 300+ pager, excluding covers (as it happens I am working on a 320-pager at the moment) I would expect the schedule to allow anywhere between 10 and 15 working days for first draft from receipt of text and images. The one I am working on at the moment has a real squeeze on the schedule and guess where the squeeze happened. For that, they are paying a bit more pp, as I am going to be working all weekend on it.
So, you see, there are too many variables to be able to give you a flat rate. With my clients, I usually negotiate a flat fee for initial design visuals (inc up to three sets of proofs), then a page rate for production of the finished book. This stage usually has first draft, then a second for editorial and layout amends from both author and editors. After that it goes to the proof reader and final tweaks are made. Last proof goes out for approval to all parties, fed back to me via the project editor, then final print-ready pdfs are made and delivered along with final live files and support (for co-eds) – making sure all relevant font licensing is in place.
Hope this helps.