General maintenance, parts and actually riding.
I build (if required) and maintain all my own bikes so no shop costs. Cables, BB's etc need changing more regularly on the MTB, I ride a hardtail so at least I don't have to maintain pivot bearings and bushings too! Servicing forks costs a bit for seals and oil. Despite what people think hydraulic brakes very rarely need bleeding, but that is more expensive, though the pads are cheaper compared to the Swissstop greens on my road bike.
Parts in general - My road bike is the stock build and I have zero interest in upgrading bars, stem, saddle, wheels etc. So the only costs will be replacements as and when needed.
My mountain bike is different, I bought the frame and transferred bits from a cheaper bike across and have been slowly upgrading it, so it's had a new saddle, seat post, bars, grips, forks, wheels, discs and will soon be getting a clutch rear derailleur and possibly new cranks (I ave a triple but only ever use the 32 ring).
A lot of clothing crosses both, I wear my bibs on both, jerseys in the summer on both (not my downhill stuff) road helmet for XC etc.
One of the main costs is riding though. I ride my road bike from my front door, zero cost except coffee and food. I usually drive to ride my mountain bike and that usually means parking charges, food, bike wash, fuel costs. All adds up. I'm lucky that I do have some local riding places. Here's some raw footage from today:
https://www.youtube.com/watch?v=BGySKhYCiyU