I can't tell you what it'd be worth to you, I can only tell you what it's worth to me. Also, I never played Dom2 so I can't make that comparison.
Following Rdonj's lead, I can compare Dom3 now to Dom3 at release, and there have been a lot of additions. Even if the devs hadn't been adding to it (as well as the community creating new maps, mods, nations, etc) I think I certainly would have gotten my $55 out of this game many times over. I still count myself as a newb around many of the guys on these forums, but between fooling around in SP and the few multiplayer games I've played (both LAN games with friends and MP on llamaserver / dom3minions) I've probably racked up a couple hundred hours playing this game. I still enjoy it.
Dominions 3, like Dom2 I believe, was never about the latest shiny graphics. It's about gameplay, a richly thematic world, and near endless replayability. The gameplay hasn't gotten any weaker in the years since it's release.
If you never play anything but unmodded Dominions maybe this won't mean much to you, but the number of mods people have made for the game is quite large. You can find links to a whole host of them on the mods section of these forums. Or you can check out the other forums that host mods, such as:
http://z7.invisionfree.com/Dom3mods/...hp?showforum=1
Some of the mods are so widely used, they're almost canon (Conceptual Balance Mod; Endgame Diversity Mod). CBM tweaks the costs of various things with the goal of changing the balance between nations and units to make those that were rarely used in MP games more viable while reducing the utility or increasing the cost of the most commonly used units / summons / items / etc. EDM introduces a bunch of new mid to end-game summons that give Tartarians a run for endgame supremacy.
Don't know if you like MP games at all, but there's a whole other side to the game that you discover when you start playing MP games. Granted, it may ruin SP for you (the AIs never seem like as much of a challenge any more once you've been pounded on by another human).
BTW, If you have any interest in MP games (or if you're curious but you're not certain about it) there are some great Let's Play threads on the SomethingAwful forums for dominions, including screenshots and everything (though now that I went looking, you might have to register there to read them - looks like they've started archiving old threads). Gives you a decent idea what to expect from multiplayer games. There are also "newb" games starting in the Multiplayer and AARs section of the forums here, as well as on the
Modding Forums.
And if you like the idea of being in a cooperative game with other players, I'm currently in one w/ other players vs buffed AIs. The AIs haven't put up the resistance that a player would have, but between the buffed AIs and the tough / unusual independent forces thanks to the Semi-Rand (Ballbarian's map tweaking program), it's been an interesting game. The game was proposed by one member of the forums here, hosted on the server of another (Gandalf - who helped us with custom settings for the game), and the map and pretenders were designed by a 3rd (NTJedi). Gandalf and NTJedi aren't playing in the game, they just volunteered to help us get it up and running because they found the concept interesting.
Anyway, I think Dom3 is a really great game; it's given me years of enjoyment. Does that answer your question?