Invasive non-native species are estimated to cost in the region of £1.7 billion a year in Great Britain (Williams et al, 2010). 

Examples of costs associated with invasive non-native species include:

• A predicted £1.56b to eradicate Japanese Knotweed nationwide (Defra, 2003). 
• £2m spent to eradicate Japanese Knotweed on a 2 hectare development site (Williams et al, 2010)
• £11m estimated to be the cost that would be required to eradicate Rhododendron from a National Park in Wales (Postnote, 2008). (Postnote, 2008)
• £13.9m of damage per year estimated to be caused by Deer vehicle collisions (many of which were non-native species) (Langbein & Putman, 2006)
• £100,000 spent by one water works on modifications to cope with Zebra Mussels (Williams et al, 2010)
• £10m estimated cost to British timber industry of Squirrel damage to Beech, Sycamore and Oak woodland (Booy et al, 2008)
• £1m delay caused to a road development scheme while waiting to treat Japanese Knotweed (Booy et al, 2008)

References

The Economic Cost of Invasive Non-native Species to the British Economy (Williams et al, 2010) (PDF, 2MB).
Review of non-native species policy (Defra, 2003) (PDF, 1MB).
Invasive non-native species (Postnote, 2008) (PDF, 1MB).
National deer-vehicle collisions project (Langbein & Putman, 2006) (PDF, 1MB)