You've got to do some serious miles to get the install cost, or kit cost to pay for itself.
The cost difference is not as great as it used to be, then add around 10-15% less economy per litre, but oil lasts lot longer.
So I suppose depends on mileage, can take cat off and never have to worry about emissions anymore though, my last car was 0.0015% co, plugs last longer too.
Diy is no prob if competent, wet kit works much better, really need a spark advance unit to get the best.
tabetha