Well, the SDK is free, and if all you want to do is some 'hobby' programming, then you'll be fine. However if you intend serious commercial development, you will need:
- Several different unlocked Symbian devices (to check for cross-device compatibility). Unlocked devices tend to come in around $400-$500.
- Application signing (as noted above) - most potential customers won't install unsigned apps as they produce dire warnings about malware on the UI.
- You may want to invest in one or more premium Carbide extensions. These are invaluable timesavers to the professional, but again, are not cheap.
One other aspect to consider, depending on your application space, is that you can develop for Symbian using gcc as a cross-compiler or ARM RVDS.
The ARM compiler produces significantly better code, but it it far from cheap. This probably only matters for performance-intensive applications though. The code produced by gcc for ARM is not bad these days.
I'd concur with the suggestion to use Qt for any new development.