The sensor and associated filters, LCD displays, firmware which allows operation of the camera and all the associated electronics to control image quality and storage of data come to mind when considering extra costs.
A film camera is only hardware that makes sure the film is correctly placed and transported. Yes, they contain a light meter, AF sensors and other controls, but their main job is to align the film behind the lens. Digitals are completely different. There's no film so you need electronics to make sense of what the sensor captured (not to mention making sense of all those 1 and zeros...lol). Think of some of the other features digitals need that film cameras don't. Things like setting color temp (white balance), storing data in RAM instead of on film, increasing sensor sensitivity (using a different ASA in the film world), noise reduction, anti-aliasing, image settings (sat, hue, contrast, sharpness, etc), multiple output formats (jpg, TIF, RAW), selectable color space options (Adobe RGB, sRGB) and the list goes on.
That you can pick up a D50/70 or 350D at their current prices is pretty amazing considering that a 2mpxl Dslr cost close to $10,000usd only 6 short years ago.
It is no shame to be "stuck" with an entry level Dslr. Matter of fact, in your case it may be a boon. Using an entry level camera should provide you with a solid knowledge base and an understanding of how to get the most out of a Dslr. It will also bring to your attention things it cannot do well. So next time you are in the market, or looking to upgrade, you will be quite sure what features/performance your next camera should have. The 350D is a scaled down 20D. Personally, it doesn't fit my hand as well, but IMO the image quality is virtually identical
Remember the old axiom; bodys come and go, but good glass lasts forever (or was it, diamonds are forever?...lol). Seriously, invest in good glass and you will most likely be thrilled with the results these entry level Dslrs can produce
Good luck,
Steve