I think the PC signs take up only 1 slot :) at least all the Reon ones do in my gallery...
about price per mb i think
MA has a curved out function for this ... starts somwhere around 30 ped ... when i was playing with my screens i first uploaded one of my drawings in low res and it took around 37 ped ... with the high resolution images now available in my gallery (Billton T3, 5G) the cost was approximatly 60 ped even tho the files were 10x the size of the low res one :)
Im not sure on how many pont do the screens and displays take tho... i do know that if u plan to post a static image or simple text its better to get a sign since fees on placing the add on it are lower :)
Click to enlarge