I would suggest using one of the tricks of behavioural economics which I personally (many time knowingly) fell into, by introducing an option in the pricing model that is meant to sway people to choose the higher price.
Having isa priced at £3 monthly
Plus without isa at: 6.99
Plus with ISA: 7.99
People will basically think that they are getting a £2 bargain moving to the last option, very few people (if any) will go with the middle price, its introduction is purely to show how much of a bargain is the 7.99!
This is called the decoy effect.