Probably about £15 for a single gauge type or £25 for a twin gauge. The twin is the better buy in the long term. The single one tells you how much is left in the bottle, and the twin also gives you a flow rate, so you can make sure you aren't wasting gas (or worse - if you are not using enough).
Local BOC place has them as a kit to convert mini MIGs to big bottles, being BOC it's £40! :eek:
I got one with my welder, I'm sure my mate whop I bought it form said they were £15-20 at a welding supplies place up the road, time to get the yellow pages out!