O2 are currently running a half price sale on their PAYG MiFi units when bought with data (the unit on it's own costs £60)
Last month they had this exact same offer and I got this for my parents as I didn't want the hassle of a contract. It then expired before I could post here but it's now back on till the end of this month.
You also get double the data on your first and every third top up.
I went for the £37.65 "Pocket Hotspot including 2GB data" as you get more data on the unit to start with.
Also Quidco tracked at £5, so it should work out to be a net £32.65.