I assume you're not in the UK then, despite stating prices in GBP?
Three's One Plan is pretty awesome, SIM only deal, rolling 30 day contract. Paid up front so no credit checks required. You don't pay, you don't use. Simples.
I have an old cable modem running DDWRT in the house, setup to act as a wireless client, with CyanogenMod 10 running on my HTC Desire HD to offer wifi AP. From there, I've got a few things plugged in, but most importantly is the old netbook running Ubuntu which has hostapd to offer a static wifi through the house, even when I'm not home. That setup is perfectly fine to have 3 PCs running BBC iPlayer in HD, a PS3 (which is a bit of a hog & has been rate limited by iptables) and an Xbox360, all at the same time, with no noticable lag. Though I'm also lucky enough to have HSDPA+ coverage at home.
Then we save a little cash by not having a TV (no subscription nor licence) and the "phone line" is replaced by a VoIP setup (cheaper calls + no line rental).
I just stated the prices in GBP because you did, and it was convenient to convert to that.
You could get a WiFi router that accepts either a SIM card or a USB 3G stick, it could be simpler than using your phone as the modem.
Agreed on the possibility of getting a wifi router that accepts a SIM/USB device. But that wouldn't help when I'm using the company laptop - especially at times when I'm doing so sitting by a river. :-)
Plus there's a plethora of other use-cases that make rocking your own portable AP very handy.