I've only played Inca a few times but I do often take commerce tree with the policy for 50% savings on roads and railroads and even with a sprawling empire I find the savings very insignificant, often in the range of 6 gold or so, so I'd assume the Incan savings wouldn't be much more than that, which isn't bad but doesn't really compare to 240 gold per lux every 30 turns, from Arabia or the Dutch.
I often build strategic roads as it is, at least by late medieval or so, which coincides with when my economy can support them and when I either have a lot of workers or worker downtime. I'd still take the Iroquois forest-roads though which require no build-time, are available from the start of the game and are generally spread out all over already, forming effective strategic networks, with maybe a gap or two that are easily filled with a single road tile.
When I play Inca i find myself feeling like I'm playing a very bland agricultural civ or something, like they just want to stay at home farming their terraces, not really teching ahead of anyone or becoming richer than anyone, with a mediocre military that likes to run from combat and often sheltering behind a great wall of mountains in isolation, which the AI is too stupid to get to you around.