For those budget minded, availing yourself of the buffets and small plates that accompany your aperitivo can prove to be a great bargain.
Here's a list by Puntarellarossa (and while on their site, check out their other Rome suggestions):
The best aperitivo in Rome: ten cocktail and wine bars to know about
And this is from Romeing (this site tends to skew towards a younger demographic - for our younger FTers reading this!):
Rome Nightlife Guide: Bars for Aperitivo