Yeah, looks like all the ones on Steam cost $19.99. Same as the base game.
I *really* don't see the need. The base game is super strong by itself, and has so much to do. More than I'll ever exhaust. But, you can get one of the three biggest expansions along with the base game for $29.99. I chose University Life, and I think it was the right decision. Even without going to the university, it adds a jock/nerd/rebel "reputation" system which allows new interactions. As a ranking nerd you can "berate incompetence", or "enthuse about video games", and more credit unlocks more interactions (and video games for the sim, and other small stuff). The main benefit to me was being able to get some idea of an NPC's personality at a glance, since you can see their cred levels.
Plus, I guess there's a university! I haven't gone yet, but I did buy an honorary degree with lifetime achievement points. Degrees grant additional perks of your choice. Dunno if it's limited to 1 or not.
So yes, clearly all the DLCs are useless except for the one I purchased. Or something. $30 seems like a decent price-point to me, so I recommend getting one of those 1-DLC packages.
(I played the base game a lot about a year ago using a friend's CD, and still had fun replaying it now)