Starcraft has rarely (if ever) been beaten in terms of variation between races, but still having a good balance. This is why it's still immensely popular as a multiplayer game (especially in the far east).
If you're looking for other RTS games, Age of Empires 2 was also massively successful - it's also quite old now, so you should be able to pick it up on budget.
There are many newer RTS games - Age of Empires 3 has been out a short while now, for example - but generally they look pretty but don't have the same gameplay as older games like Starcraft and AoE2. It's up to you whether you're more interested in eye-candy or gameplay!
AoE3 has a playable demo, so you can download that and give it a try. Rise of Nations is also worth a try - it's been out a while now so should be cheap, but a lot of people liked it (I found it a bit too complicated - too much to do and not enough time!).