If rumors are to be believed, PlayStation Now may cost $5.99 and below for individual rentals.
In a report by VG24/7, a photo of concept art posted to Gaikai's site which featured prices next to Uncharted 3 and Far Cry 3 priced at $4.99 and $5.99 respectively. Although these have yet to be confirmed officially by Sony and the pic has since been removed by Gaikai, the chances that these are within range of the actual price aren't inconceivable.
PlayStation Now was announced in January in Sony's keynote at CES and is scheduled for a Summer release. Sony has handpicked a handful of beta testers to test the product in the meantime.