Just before Covid struck in 2020 I did a benchmarking study for the UK racing club I belong to, 15% of the 400 active racers kindly supplied a lot of data but that didn't answer a frequently stated view among older members -" racing was so much cheaper in the old days"
However one guy had kept his 750F racing records from 1975 and as 750F is still active in largely unchanged form that question could be answered albeit from sample of one. Basically if you adjust costs from 1975 to 2019 by UK CPI the cost is roughly the same. If you adjust by wage growth its cheaper now.
However there was a very distinct shift in the individual cost elements. Basically the use of production and heavily controlled engines plus control tyres reduced running costs by a lot whereas the external costs of licences , entry fees and personal safety gear etc. had skyrocketed.
Overall today in UK club racing it seems that "non controllable" racer costs - entry fees, licences safety gear and travel are 40% or so , within limits the race car you choose isn't so critical.
Am amusing side revelation was that the biggest capital cost for about1/3rd of racers isn't the race car it is the motorhome!. People with $10,00 cars often tow them behind £30,000 motorhomes. part of the reason is two day meetings plus wanting hte family to come along.
Yes, I see the motorhome becoming more prevalent on a USA site I frequent. My mind boggles ...
Not surprising safety gear is a bigger proportion of costs, considering what safety gear was available, let alone mandated, back in the day. But that is generally only a one-off, should be amortised over a long period - unless you are in the habit of testing its efficacy. Helmet, clothing, boots, gloves, underwear, HANS are more of an investment than a running cost. I am using mostly the same gear I purchased over 10 years ago, it is still servicable and current if not pretty, though I have replaced boots, gloves and balaclava over the last few years as wear has taken a toll.
Entry fees are a big item, fuel, brakes and tyres not far behind. Petrol for the towcar, perhaps one or two night's accomodation for some meetings. Engine refresh every few years, depending on usage, I've been lucky in that my last couple of refreshes only required bearings and gaskets, no expensive stuff!
It is not a sport for the poor people, but careful choice of vehicle and class/events can produce decent bang-for-the-bucks. OTOH if you have deep pockets, have at it!