If you are looking for the most affordable digital camera, does that mean you are looking for the cheapest camera you can find? I tend to think not. If you wanted cheap, you would have said cheap. Affordable suggests, you have some kind of budget in mind. I would assume that it's probably a small budget, but a budget none the less. For around $150 you can get a pretty respectable digital camera. But that may be a bit high, so lets start at $150 and work our way down. You may be surprised at what we find.
In the $100 to $150 range, were able to find some very good cameras with a smidgen of the latest technology thrown in. Image stabilization and face recognition are great features to have for this price. The image quality here will be far better than anything your cell phone camera can do, and a vast improvement over disposable cameras. The Canon PowerShot A570IS would be my first choice, as well as many others judging by it's inclusion on the best seller list on some of the big online camera retailers. The Panasonic Lumix DMC-LZ7K and several of the Kodak EasyShare models can be found in this category as well.
The $50 to $100 range is getting quite affordable for anyone, but there are sacrifices to be made in image quality and features. The Kodak EasyShare C613 still has a respectable 6.2MP and 3x zoom. A perennial best seller, and backed by one of the oldest camera manufacturers in the business. The Nikon Coolpix L11, and Canon Powershot A460 are worthy contenders to look at here as well.
Buying a decent new digital camera (that's not a toy) for under $50 is pretty much impossible these days. Prices have dropped quite a bit, but not that low. The good news, is that if your willing to look into used cameras you can find plenty of great deals. The usual auction sites will have lots, but some of the bigger online retailers carry used cameras as well. Just make sure their return policy is good, before buying anything used.
The most affordable digital camera, is of course the one you already own. Pretty much the only cost associated with that one is batteries. Since so few people develop a lot of pictures these days, even developing costs are hardly worth mentioning. Don't ignore any used and discontinued cameras you may find. With so many new cameras coming out every day, there are plenty of good deals to be found as people sell of their old ones.