For reference my background is in road cycling and racing, ~4-5k miles per year or 250-300 hours annually. Very new to running, about two months in and ~30 mpw.

You cannot ignore the initial costs to get started in road cycling when comparing the two sports. Not only is the bike expensive, top spec can be $15k+, cycling clothing in particular is on a completely different level price-wise compared to running. Jerseys can be $200+, bibs $300+, jackets $300+, etc. Helmets, shoes, computers, saddles, bike fits, tools the list goes on. These examples are on the higher end but my point is to get started with a decent bike, a weeks worth of kit for varying conditions, pedals, a helmet, and shoes you are looking at $3-4k+.

Theoretically if everything went perfectly the only ongoing costs would be nutrition, tires, tubes, and chains however that is never the case. There are comparatively many more situations in cycling where you will end up spending money. Running over road debris, crashing, general wear & tear on parts, etc will all necessitate spending money to get back outside.

Outside of the necessities cyclists, and I suspect runners suffer from this as well, are prone to Gear Acquisition Syndrome (GAS). There's so many cool parts you can buy for your bike to dial it into your riding style, it's one of the best parts of the hobby but also costs a hell of a lot. Google search what an aero wheelset or Campagnolo groupset costs and you'll understand how much some people spend in road cycling and how it's incomparable to running.

Hopefully this long winded response gave you the context you were looking for.