Paris doesn't have fare equivalency for the metro and RER. The metro is a single fare, and the RER is a zone fare. You pay about a dollar more or about 25% more to take the RER from Gare de Lyon than you do on the metro.
It's my understanding there's fare equivalency within the walls. As long as that happens with a future GO RER+TTC faresystem unification within 416 (just like Paris RER+metro within the old walls of Paris), there can still be zone fares for longer trips going from 416 outwards to 905.
Some more info about the Paris RER fare system --
http://www.parisnet.com/metro.html. You DO sometimes have to pay an increment... It depends. But there's easy transfer between RER and Metro. That's what GO needs over time.
This may mean we'll eventually get a tapout system on TTC (Not Vancouver's flaky system which uses server communications at tapout, but same performance as the quick tapout system they use at London Underground) as part of faresystem unification eventually.
I got tweet replies from TTC/Metrolinx that strongly suggested new Presto turnstile design is going to be using Presto at the top, rather than the front -- similar to London's system. In theory, this allows TTC to easily enable a tapout system eventually. After all, the chip used in London Octopus cards and Ontario Presto cards, is identical, so it's technologically implementable. Once that's done, we may tap-in on a TTC, then tap-out on GO RER. And tap-in on GO RER and tap-out on TTC. The new SmartTrack-Crosstown interchange station at Eglingon, would have no faregates in between. Likewise, there will probably be no faregates when you transfer from SmartTrack and a Kitchener GOTrain, because they're forced to recycle the same GO platforms on the same GO routes.
The writing is so obvious on the wall that
THERE WILL BE FARESYSTEM UNIFICATION, FULL STOP
*** Gateless interchange stations between GO and TTC is actually happening at Eglington
*** Share some of the same GO platform between SmartTrack and GOTrains. You'll be standing on GOTrain platforms when you step off SmartTrack at some platforms.
*** Metrolinx said SmartTrack is a GO RER with an Eglington spur
*** TTC accelerating full Presto deploy to 2016. This makes it possible to say goodbye to Metropasses/tokens by SmartTrack complete.
*** Suggestive information we'll get a similar style of tap turnstile as London Underground, which uses high-efficiency high-speed tapin/tapout (presto reader on top of turnstile: You can literally run while tapping!). This enables TTC to decide to use tapout in future.
Catch any SmartTrack train. Get off at any platform. Due to GO platform reuse, guess what -- nothing stops you from hopping into a GOTrain that passes after the SmartTrack train! There's no way to faregate the GO platforms if you're already standing at the same platforms. You're not going to be able to install extra platforms at all the GOTrain stations. And we aren't going to use short UPX-style platforms (SmartTrack trains will be longer than those). Politically & economically, the pressure is towards fare unification if you go between only 2 or 3 GOTrain stations within 416, and only get charged increment when you go longer distances on the GO system. Electric GO RER lower costs, Metrolinx said the operating model of SmartTrack looked good even at TTC fare, and GO RER adds enough capacity to pay more operating costs at lower fares.
It's happening, whether you like it or not. We might have gradualness (as GO RER slowly comes online) and a complex hybrid (but no more hybrid than Paris RER) but the day is coming where there's no more faregates GO and TTC (beginning with the SmartTrack-TTC interchange). Want to bet your mortgage?
TL;DR: John Tory already de-facto promised you'll be able to tap-in at GO RER/SmartTrack Gerrard Square and tap-out at Wellesley TTC -- and pay only the same TTC fare you'd normally pay to go from Union TTC to Eglinton TTC. No faregates between GO and TTC at the Crosstown endpoints. Also, you are already standing on some GO platforms when you get off a SmartTrack train - how will you resolve that ambiguity. There you go. That's the start of faresystem unification.