CBT is almost impossible to fail unless you crash into a bus or something.
CBT cost varies from place to place, for e.g. in my home town alone the price varies from £70 upto £85, this must be retaken every two years until you get your full liscence (Takes about 5 hours to do) so I would really considder going for your full liscence before your first CBT runs out, since you have so much experience with the car passing your bike test should be easy and including the cost for lessons shouldnt be much more than passing the CBT again.
My insurance as a learner on my 100cc scooter costs £300 per year (thats including the subcharge for paying monthly) and road tax is £15 per year, though since you hold a full liscence and have done for a few years, aswel as beeing a fair bit older than I am
, im sure that will be cheaper for you.