Based on my experience from 10ish years ago, it was about £100 for the CBT which is a one day course allowing you to ride anything up to and including a 125cc (with the caveat you can’t carry pillion, use motorways, and have to have front and rear L-plates).
This is where it might have changed, but back then you had to wait until you were 25 to do direct access courses 1 & 2 to allow you to ride any bike anywhere and carry pillion. 1 being a short test on a closed area setup with cones to do manoeuvres and stuff on, 2 being a longer on-the-road test with an examiner following you on a motorbike and giving you directions through an earpiece.
Can’t remember the costs involved, sorry. But it wasn’t much. The route I took meant I used my 125 for a year commuting to get experience then when I turned 25 I felt much more ready to do DA1 & 2.
They are intensive courses available for those over 25 who have done their CBT but want to go straight into being able to ride a bigger bike, I would expect this to be more expensive though.
I guess it would depend on whether you need to use a motorway to get to and from work? A 125 would be powerful enough for other road use, and comes with the benefit of restricting any mad urges you get to go fast and do sick wheelies etc.