All the games I mentioned have some age, meaning that you can get them cheap if you manage to find them. The exception is ofcourse the later games from Bioware and Obsidian (NWN2, Mass Effect, Jade Empire etc).
I would put the games I mentioned in the following categories since I do not know what kind of RPG's you enjoy:
First, if you simply want diablo-style hack & slash (like Loki/TitanQuest), I am a bad person to ask. I might be able to shake my brain for some games (Dungeon Siege, Darkstone, Silverfall, Titan Quest, Sacred, NoX, Throne of Darkness etc) but thing is, I do not know if any of them are good for you since I just cannot handle a game without a good story to boot. You are probably better asking someone else in that case.
If you enjoy diablo-style but want more story and quests like Arcanum though, Dungeon Siege II and Divine Divinity are good picks since they have diablo-style combat but with a great amount of background story and a great amount of quests to do. You might also wish to check out Icewind Dale that have a greater focus on hack-n-slash without giving up the story.
If you are a tactical gamer who want to plan your next move and have turnbased combat like Fallout, Temple of Elemental Evil from Troika and Jagged Alliance II might be good to have a look at. You might also wish to check out a game like Wizardry 8 and the games from Spiderweb Software since they are very strong in tactical gameplay.
Common with the four companies I mentioned; Black Isle, Troika, Bioware and Obsidian is that they have great focus on building and roleplaying your own character, often with greatly written companions, stories and dialogue. They offer a great roleplaying experience and often deal with deep and complex storylines. As a downside their games are often quite linear. Still, these developers are among the most renowned rpg designers out there.
If you are the kind of roleplayer who enjoys freedom and want free-roaming, a wilderness to explore in your way, just like in Fallout, Arcanum and Morrowind, then the Gothic series and Divine Divinity are great for that. They allow you to do in any direction you want. The first Baldur's Gate was also designed in that way. You might actually wish to have a look at S.T.A.L.K.E.R if you can handle a first-person shooter. It IS a first-person shooter, but it also have a good amount of RPG features and it is free roaming. Most people on RPGWatch shun Oblivion (there's actually a hate thread about it), but it is a free-roaming RPG. I think the reason it's shunned is for being so dumbed down, which we old and grumpy veterans do not like, but people who aren't used to hardcore RPG's tend to love it. The last two (STALKER and Oblivion) might not be budget games though however since they are fairly new.
Deus Ex and System Shock II are first-person roleplaying games, both with a deeply immersive story. Deus Ex have won numerous awards as "best story ever written" etc and System Shock II often end up high on gamer's "best game ever" list. I do not know if they are made by the same producers, but their interfaces have a lot in common. Deus Ex II was in every respect worse than the first, but it's still a good game.