And in 1969, high octane gas was $0.25/gallon. Straight conversion of 1969 to 2015 dollars doesn't work, because a lot of the cost of modern cars is not due to the cars, it is due to gov't requirements (ABS, traction control, tire pressure monitoring system, stability control system, etc.)
As for truly affortdable muscle cars, with comparable performance, just get a standard 5.7 Hemi Challenger. It should be in the $25-30K range, or a standard Mustang GT, same price range.
And to be frank, here today, in 2015, it really doesn't matter that a 1969 Superbee was $4K, or $3.5K, because NO new car costs $3-4K. I have pointed out the performance cars of today that could be had for $25K. I used the top end muscle cars as an example due to their being the only ones that could come anywhere near being compared to a Hellcat, and they would still lose. A ZL1 Camaro (one of 17), or a Boss 429 Mustang (63 or so) would lose. As for affordable performance, would a Mustang GT beat a 440 Six Pack Superbee? Maybe yes, maybe no (and I am talking about straight line, 1/4 mile drag race, both cars being stock, but broken in). Point is, performance would not be far off, except when the road is no longer a 1/4 mile drag strip, the Superbee becomes a rapidly receding image in the rear view mirror. Is the Mustang GT as "raw" as the Superbee. No, no modern car can be, partially due to gov't regulations, partially due to customer expectations. Is the Mustang going to be more comfortable, safer, stop better, turn better, and will it do it in air conditioned comfort, and get better gas mileage while doing it? You bet. Will you have the feeling that you are driving something special, that can hold its own against most cars? You bet. Is it as good looking? Hard to say, as beauty is in the eye of the beholder, and comes down to personal preference.
Yes, you would be paying more for a 392 Challenger than the equivalent 440 Six Pack; but you are also getting higher performance, amongst other things. How much did a Hemi Road Runner, Hemi Cuda. or Hemi SuperBee cost back then? I know a Hemi was between $500-800 all by itself for a base car, and the 440 Six-Pack was about $240 over the base car, so add another $300-460 to the SuperBee's price, and work it out. And in the end, today's 392 Challenger is still faster.
As for the "How many of today's muscle cars do you see?" question, this is not 1969. How may ricers (Subaru WRXs and the like) did you see in 1969? How many European cars (Audis, BMWs, Merecedes-Benzes, Jaguars, etc.) were being driven around in 1969? How many jacked up Ram Diesel trucks? All of those were non-factors in 1969, because they were simply not available. In 1969, today's guy driving a jacked up, soot spewing diesel Ram truck very well may have chosen to drive a 440 Six Pack Superbee. However, today, he drives the Ram truck, spends $20-30K on jacking up the suspension, putting on oversize tires and wheels, and chipping/tuning the Cummins diesel. If this truck, and the aftermarket for it, were not available, he would be in a 392 Challenger, or a Boss 302 Mustang or Camaro SS. The ricer driver may have been in the same type of car. In my neck of the woods, there are tons of such cars; replace them with any of today's muscle cars, and the ratio would probably not be far off from what you had in 1969. The percentage of people who want to drive something special really doesn't vary from generation to generation, as an opinion, what does vary is what types of "special" cars are available to fill that need. In the late 1960s-early 1970s, it was muscle cars, only. Today, there are many, many other choices.
In 1969, you bought a muscle car because it was the only game in town if you wanted something special to drive. Today it isn't, so what was a muscle car only market has become a very wide range of vehicles. In Calgary, there are more shops that specialize in diesel high performance than restoration and hot rod shops combined. The trucks that go in are in the $40-80K range, and come out with the added $20-30K in add-ons. Obviously, those drivers could have gone with a Hellcat Challenger(if they weren't sold out, and if they were available here)/Z28 Camaro/GT500 Mustang for the same money, but chose not to. This has nothing to do with the affordability of the cars, as obviously, the money is there and being spent; it has more to do with what people prefer to drive. I don't understand why people would buy a diesel pickup and modify it, but there are many people around doing it, so what do I know. All of the expensive vehicle owners,(diesel truck, ricer, European import) are lost modern muscle car sales.