For installation from scratch, usually add 50% of the equipment value. That is a
very rough rule.
Sometimes it is much less, e.g installing a Mode S where there was a same-sized Mode C and the antenna remains, the installation cost can be down to £200. This is partly because the avionics shop has made £500 on the trade discount on the supply of the unit
Most situations where somebody pays much less than normal on major changes involve some unusual scenario e.g. a very friendly avionics man installing customer issued parts and perhaps paid cash in hand. You can buy avionics for far less than UK prices from the USA from
here for example but most UK avionics shops won't touch the kit with a bargepole because there is no trade discount on it. The only cases I know of where this worked was either with a friendly avionics man, or in the context of a really big refurb job.