Sorry, was traveling and only saw this now (thanks Gergo)! Generally many places are visa free to us, and if you have Schengen, UK or US visa valid (and have actually traveled to those places at least once on that visa) you can come visa free to Serbia. Visa is also generally good to get, but embassies don't exist in all countries so sometimes you have to ship your passport (from Philippines to Indonesia for example), so do plan accordingly. Happy to help write an invitation letter, but more important for the visa is having booked accommodation, there's cheap hostels walking distance from the office, I'm happy to send links.

I don't but it's supposedly much easier than to EU member countries

I mean low-cost is always relative, it definitely won't be as cheap as, let's say, some Eastern European cities, or Bangkok. But still, relative to most Western capitals, the overall cost of living in Berlin is quite low, especially if you rarely eat out. Rent is the only thing that's become pretty high in recent years but if you are lucky/utilize some connections you still might be able to find something reasonably priced. (I know several people that pay like 400-500€ a month for a room in a shared flat.)