The article evaluates the cost of upsizing images. A starting point for this is the variation of stock photography prices with the image size.
Why stock photography?
A quick definition of stock photography might be helpful here. The Wikipedia entry says that it consists of existing photographs that can be licensed for specific uses. Book publishers, specialty publishers, magazines, advertising agencies, filmmakers, web designers, graphic artists, interior decor firms, corporate creative groups, and others use stock photography to fulfill the needs of their creative assignments.
So, stock photos are raw, unprocessed images, mainly. Artistic intervention is present only at a base level, the photographer. This is why they give the best approximation of the actual value of a photo.
Stock photography sites
There are many large stock photography sites online. A quick search on Google returns popular names like Corbis, Fotosearch and Comstock. These businesses prosper by selling photos at different sizes, as the client specifies. Prices for these sizes vary to suit everyone's needs. For small images the price is low, but these can be used only on websites and other mediums that don't require great resolution. The expensive large sizes are suited for printing, displays, posters etc.
Price vs. size
Take a look at this plot first. A more detailed view of the data used can be found here. The blue line represents the average price of an image versus its size (in megabytes). The red line is the best linear fit. This gives a trend for estimating prices for larger photo sizes.
How the data was collected
The top 3 popular stock photography sites were considered. Photo sizes and their respective prices were gathered for several types of images on each site and entered into a table. The average price for each size was computed. This average price was plotted against the size of the images. As a side-note, if you're interested in how many megapixels the size in megabytes represents just divide by 3.
The resolution of the photos can be derived from their size (in MB) like this:
640 KB - approx. 640 x 480 pixels; 8.9" x 6.7" at 72 ppi
2 MB - approx. 1024 x 1280 pixels; 14.2" x 17.8" at 72 ppi
14 MB - approx. 1700 x 2550 pixels; 5.7" x 8.5" at 300 ppi
32 MB - approx. 2800 x 4200 pixels; 9.3" x 14" at 300 ppi
50 MB - approx. 3400 x 5100 pixels; 11" x 17" at 300 dpi