Haven't done many Audaxes for a good few years but when I used to do them I used to pick events solely because I could get to them by train.
Not owning a car (at that time) would have meant renting one and, although that might have worked out as cheaper than train and extra hotels (I don't like the idea of driving straight after finishing, and would prefer to rest in a proper bed) in the end I don't agree with one more car on the road for something which isn't necessary.
Plus I'm based in London so it was quite easy to get by with trains for lots of rides.
I think I only did 5 Audaxes where I got to the start in a car (two were riding with friends and I couldn't convince them not to drive, another was a lift from a yacfer already driving, one was a lift from the organiser in return for helping set up, and the last was LEL'09 and that was more my wife wanting to wave me off).
Most of them just took planning in advance and required stays before/after, i.e.
Elenith: Train to Kiddiminster the night before, train back the day after the ride, 2 nights of Travelodge
Trains were usually £20 each way advance fares, TL £40/night sometimes split with someone if I ended up sharing the room
BCM: Train to Bristol on the Friday, train home on Monday morning (and straight in to work!).
Think I paid £25 in total for the trains (advance fares again) and £40/night for the Travelodge
MSG300: Train to Brum night before, hotel before/after, train home.
Knowing how to get the cheap advance train fares really helped keep the cost down.
Same for K&SW600, Snowdon & Coast 400, Border Raid 600, and a bunch of others I've long since forgotten.
More importantly, there are a bunch of rides I would have liked to have done but just didn't work logistically. They would have meant leaving work early on a Friday, or not being able to get back in time on Monday morning. Having to burn one or two days holiday for cycling would have dwarfed the entry/train/accommodation costs.
I'd only do hotels before/after for a 300 or longer. Just no point for a 200 as there are plenty local enough (or just a DIY or Perm).