Bus travel Belgrade, Sarajevo and Mostar

Someone has seriously misinformed you about travel between Belgrade and Sarajevo. It's about 200 miles by road, and you'll surely not find a taxi to do that run for 25 euros. A bus, possibly.

Rome2Rio.com is a good place for a first look at transportation options between challenging destinations. It usually provides links to the appropriate bus or train companies. It is not safe to assume that the specific schedule and price information on Rome2Rio is accurate. When necessary, it gives estimates of taxi cost. Again, I'm sure not totally reliable, but at least it helps you distinguish between a likely 10-euro fare and a possible 100-euro fare.

BusTicket4Me, a website I know nothing about, shows a few buses a day between Belgrade and Sarajevo, taking about 7 or 8 hours. I don't know that I'd want to buy an online ticket from them, but for international bus tickets it's usually a good idea to buy in advance, because there aren't that many buses going across borders, especially in the Balkans. When I arrive in town, I always stop to check on the schedule for the bus I anticipate taking out of town. It's important to pay attention to the day of the week and any looming holidays, because those can affect the bus schedule.

As I recall, the bus station in Belgrade is near the train station and it's a fair walk uphill to the heart of the historic district. I haven't been to the other cities in 20-30 years so can't help there, but any guidebook should have that information, and you should have a guidebook for Balkan travel.

I just returned from a trip to the Balkans and had a great experience with Taxi Pajo. I was traveling with my partner from Sarajevo to Dubrovnik, and the cost for the trip was 140 euros inclusive (for two people). It's more than the bus would have been, but the experience was well worth it. I'm not sure what their rates are from Mostar to Dubrovnik, but I'm sure it would be less, as the company is based in Mostar. Our driver (and I believe one of the owners), Mili, was great to talk to and knew about a recently opened border crossing that saved a huge amount of time. Highly recommended!

We have also taken local taxi/minibus services from Sarajevo to Mostar and back. They are the new solution, and for a couple or small group better than the bus. Any Sarajevo taxi driver will also be happy to give you his card and arrange to drive you to the bus terminal (be aware there are two bus terminals in Sarajevo, one for Republika Srpska and one for Bosnia). We waited at the Bosnian bus terminal for our minibus to Mostar to pick us up. By sharing the cab with a few others it can be cheap.
There are many long distance bus lines crossing Europe now. Look for an express bus from Mostar to Dubrovnik, a long way on a good road network rebuilt since the war down the Neretva valley, home of Opuzen and the best mandarin oranges in the world.