Champions of Norrath and Return to Arms aren't online only expansion packs, like the PC version has had. They're both standalone action games, and incredibly fun to boot. They do have online modes, if you're interested in that sort of thing. I've had 'em both since they came out, and I remember we tried to set up some 2-4 player raids with some of the forum members, but I can't recall if we actually did.
Also intriguing is the fact that you can play through Norrath on easy, use the same character(leveled, stats, items, etc) to play through normal, and once again use them to play through hard difficulty. Then you can transfer that character over to Return to Arms and start there. Good times. You can actually exploit the hell out of Norrath's inventory system and amass a large sum of money, plus you can dupe items and trade them to friends or what have you. RtA killed that exploit dead. I've not explored the possibility of a similar exploit existing.
Norrath and Return to Arms were developed by Snowblind Studios. The reason you see a similarity between them and Baldur's Gate is because Snowblind did those, too. They also did the semi-recent Justice League Heroes game. Untold Legends (all 3 of them) were done by Sony Online, most likely with what they learned through the dev process of Norrath/RtA. As a result, they lack a bit of that 'polish', but are still fun games in their own right.
Personally, I'd love to have both Baldur's Gate games. I know Dark Alliance II has been a money commander ever since it came out. $40-$50 is reasonable, I think, provided it's in good shape. If you're digging these games at all, you're going to get your money's worth.