Depends on what you want.Also, most importantly how much is it for a pint over there?
- The cheapest I know of in the wider city is 115 RSD (84p).
- The cheapest I know in the city centre is (I think) 190 RSD (£1.40).
- Average in the city centre is around 250-280 RSD (touching on £2).
- For a decent micro-brew, expect in the region of 350-420 RSD (or considerably more for something imported like Brew Dog - you're looking at 650 RSD/£5 per pint for that).
Beers (industrial lagers) are generally around the 5.2% abv mark. Most craft/micro beers are upwards of 6%, with some around 7.5%.
On Skadarlija, you'll be paying 300-350 for a bog-standard Serbian lager or something international and crummy like Amstel or Bavaria, most likely in a plastic glass post-match or if a crowd are gathered. Heineken is always more expensive, by roughly 20-25%.
From local lagers, I drink Zajecarsko, and will drink Niksicko (a Montenegrin beer, but brewed here) and Lav. I tend to avoid Jelen, but some people like it. Nektar (from Bosnia & Herzegovina), although less widely available, is also decent.
Like I mentioned earlier, Gunners is the place to go if you want to try local craft beers and remain close to Studentski Trg. A bit further away (but also in the centre) is Samo Pivo (lit. 'Only Beer'). It's got the biggest beer menu, but prices are a bit heftier, service can be rubbish, and there are no drinks except beer (not even soft drinks, from what I remember).
One more bit of advice for the drinkers: If you want to drink rakija, go ahead, but I'd advise you not to down shots of it. It's definitely something for sipping (possibly with a glass of water alongside), and not for an empty stomach, either. If you're eating a meal, ask the waiter for advice on which rakija to drink before/after food - they generally love to talk about it and are quite happy if a foreigner has a genuine interest in it, rather than sees it as something like tequila or vodka.
On the subject of food, it can be a nightmare to get anything after midnight on a weekday, so if you're planning on pulling a late one, make sure you fit in a meal at some point, because there's no chance of a 2am kebab.