In that case, you have to consider issues such as the quality of the results and the time it takes you to achieve them as more important factors than initial cost. Intial cost counts, yes, but initial cost is amortizable.
Animoto has been mentioned, and it's probably the very cheapest and easiest way--all factors considered--to jump into producing a high-quality product that can be easily sold for 'way more than it cost to produce. And Animoto is not particularly expensive--you can spend more money going to a movie. One month of Animoto Pro service--all the videos you can make in a month--costs $39. Even if you only did one a month, the sale would easily pay the cost compared to the time you'd spend doing it with a free product.
You're not going to find a free or cheap application that's going to produce the high level of product as Animoto that won't take you hours and hours and hours--your time is worth money.
If someone intends to make slideshows a staple part of a product line, I'd strongly advise investing in the latest version of Photodex's ProShow Producer. It has a wizard that can crank out a decent slideshow nearly as quickly as Animoto, but is totally customizable in every way, and it also has some very nice commercial production perks, such as watermarking (as you see on the lower corners of television stations) and branding (showing your logo as the show loads, for instance).