The bonus is 10% to 25%. I agree that it won't destroy the economy, but I was throwing the AI a bone. I think that the cycles to reach that level is set WAY to high. Might I suggest other milestones for the game...
5 cycles to reach path
15 cycles to reach road
30 cycles to reach highway
A simple computation of a caravan between capitals and the time for upgrades...(Note: this is assuming your caravan doesn't get destroyed for any of the trips, cause then you start over)
Scenario 1, connect two cities with their capitals... a reasonable (small) distance to this would be roughly 20-30 tiles. For ease lest us say 24 tiles. This means that it will take 12 turns before a caravan has completed one cycle. Times 100, would mean that it would take 1200 turns before you have a highway after you started with the caravan (not the first tech in the game)... (Edit: Since there are two caravans on the same path it really is 600 turns)
Scenario 2, suppose that you have two really close capitals, then say you are roughly 8 - 12 tiles away, let us go with 8 for cheapness... Then it will take a minimum of 4 turns per cycle. Thus, at 100 times one has 400 turns before you get a highway. (Edit: Since there are two caravans on the same path it really takes 200 turns)
the modified numbers Scenario 1, will take 360 turns... before highway (Edit: Really 180 turns as there are two caravans on highway)
the modified number scenario 2, will take 120 turns... before highway (Edit: Really 60 turns as there are two caravans on highway)
I find that Scenario 1 is the more plausible version, but even at 60 turns it would be nice to see a highway in at least some game.
For those that want to exploit this, the count starts everytime the caravan returns home... hence you can move a caravan and move it home several times in a row