Do you have any rough price estimates for the following options?
a) train Athens-Kiato + Greek Railways (OSE) bus to Patras would be roughly 15 euro or something like that, ferry companies as well offer their buses to bring passengers directly from Athens to the ferry terminal in Patras, guess price is more or less the same. Then price for a ferry Patras-Bari/Ancona can be found
here (it's just one of the companies), while Jadrolinija Bari-Dubrovnik prices are
here, Ancona-Split would be slightly cheaper
b) bus from Athens to Tirana hardly will be more than 40 euro, then another 5 or so euro to Shkoder and another 5-10 to Ulcinj. And something like 10-12 euro Ulcinj to Kotor/Herceg Novi + 10 euro from there to Dubrovnik
c) Athens to Thessaloniki by train (from 19 euro if purchased in advance), then Thessaloniki to Skopje by bus roughly 20 euro. Price of a ticket from Skopje to Montenegro depends on bus company and final destination, but let's say from 25 (Podgorica) to 35 euro (Kotor/Herceg Novi) + another 10 euro from there to Dubrovnik
d) well, for instance as of now on August 11 Serbian JAT Airways offer Athens-Belgrade-Dubrovnik for 180 euro with overnight stay in Belgrade, on August 12 you can get Athens-Belgrade-Tivat for 160 euro or Athens-Belgrade-Sarajevo for 160 euro on August 11 etc.