I would think that just about anything that Canon comes out with that is at 40% to 50% of MSRP of the current supertelephoto lenses would be classified as "affordable". Affordable is a relative term in this case.
I don't know what Canon would make the consumer give up from the top of the line lenses to the "affordable" lenses. One stop of light? Two stops? Two mode IS instead of 3?
Would people buy a 600mm f/5.6 at say $4500 USD if it offered 80% of the IQ of the 600 f/4 II?
I can't realistically see Canon offering something that gives 80% of a 500/600 f/4 VII at 1/3 of the cost of one or less.