Boost gauges are available from £20 for a simple gauge to £150 for a Defi electronic.
Personally I would go for the new stepper gauges which are basically rip-off's of Defi and cost about £42 or £45depending on 52mm or 60mm size
these can be found at
http://www.rspec.co.uk/index.php?main_p ... 20a&page=2
there are some other Defi types but can't think of their name...it's in a mag I have so will dig it out.