Summary: I think your best bet is to look up the price for a slightly shortened trip on Voyages-SNCF, from the first train's first stop to the last train's next-to-last stop.
You can find non-TGV prices on Rail Europe as well. I just went to http://raileurope.com/, entered Paris to Grenoble and clicked on “check fares”.
As you can see, the cheapest fare if you book now is slightly cheaper than the TER fare (which you can get on an open ticket, but only at a station, not online). However, I don't know how reliable this far is, especially since I didn't even manage to display it in EUR. Rail Europe tends to have a markup; I'm not sure if this markup is consistent between modes. On Voyages-SNCF, there indeed seems to be no way to select only trains without compulsory booking (TER and Intercités, not Teoz).
The Intercités website doesn't have rates, but anyway there aren't any Intercités involved in a trip between Paris and Grenoble, only TGV or TER. You can look up the price of TER on each regional website, accessed from the TER site front page. It takes a lot of clicks and it's in French. Eventually you might be able to find prices for each leg of your journey (I'm not even sure of that, for trains that cross several regions there may be cases where you can't know the price for a span that crosses a region border). Rates are degressive, so the price from your origin to your destination will be slightly less than the sum of the prices of each leg.
You may be able to get a rough idea of prices if you split your journey into several legs for which TGV doesn't make sense. For example, you can see from the schedules on the DB website that the first stop after Paris on the Paris to Grenoble by TER trip is Sens, and the last stop before Grenoble is Voiron; if you ask Voyages-SNCF to go from Sens to Voiron, you'll get prices for the TER-only trip. Rates are degressive, so the price for Paris to Grenoble will be only a little more.
Sorry for the suckiness of SNCF's web sites, it's very well-known in France.
Your best bet to find prices is at a station. Ask an employee, as the machines aren't always able to find trip possibilities that aren't the fastest. In any case, for a trip where you don't book in advance, you'll often have to buy your ticket at the station.