All BOLS liqueur is on a 2 bottles for £10 in Asda.
My local store had:
Creme de Cassis
Creme de Menth
Creme de Cacao Brown
Creme de Bananes
This was from memory... there may have been others.
Not sure if it's the cheapest around but a quick search shows online stores selling it for around a tenner for one bottle.
Their cocktail sight looks pretty interesting :http://www.bolscocktails.com/home.asp