501b XML-U2 (£8)… lid mount(£1.30)… charger(£4) and adapter (£1.80)… used laptop cells (free).
You also need some foil to improve the heat sinking, and some electrical tape to make the cells fit snugly… and I use a small rare-earth magnet on the -ve end of the 18650s as mine are unprotected so a bit short. If the cells manage to move around they will interrupt the current which will cause the torch to change modes – this can be alarming as it is most likely to happen when you hit a really rough patch! But a torch on your lid is ‘stabilized’ so you will probably be fine without.
I’d also budget for a timer for the charger… that way you can set them to charge for e.g. 6 hours and then stop. I think single cell charging is pretty safe, but I would not like to leave mine on for 48 hours just in case!
Additional lights are £8-£15… e.g. a C8 XM-L2 for you bars.
It’s a very cheap way to get into night riding… and once you have cells and a charger additional lights (or a yearly upgrade to the latest emitter) are very inexpensive.