Ok let's say I want to retrieve a simple number from a site, and this info is retrieved on-demand. How much will this cost, and if possible, how much for a json output?
How are you retreiving the info? The information displayed is loaded after the HTML, so in essence using Google spreadsheet's XML function won't work. You'll have to use full page scraping.
Can you give me the url to have a look ?
You mean full page scraping then looking for div/span classes ? Still possible.