the most expensive addon for me was a set of scorpion slipons i bought from someone on this forum. the rest have been pretty cheap (mostly official aftermarket RE parts from ebay, new seat, bar end mirrors, sump guard, led headlamp from aliexpress, piranha flyscreen, usb charger from hitchcocks, etc, etc, etc). doubt i've spent more than 1000$ CAD so far.
this bike doesnt need much alterations for my needs/tastes.
would i spend more if i knew a local tuner who could help with the whole HP increase or possibly 865cc kit? maybe. but dont know of any and its not in my ability to do this myself so ahh well.