Of course there is huge variety of restaurants in Florence as well as in Rome. In both cities there will be tourists in all the restaurants in the historic districts, too.
But to give you a rough estimate, "middle of the road" restaurants, frequented by both locals and visitors, including house wine and water, about € 30 to 40 per person. Close to major attractions some more.
These places might not actually be called Ristorante but Osteria or something else.
Of course you can go up the ladder. A really nice restaurant can easily reach € 100.
On the other hand a simple tourist menu (seldomly to be recommended but you might be lucky) can sometimes be had for less than € 15.
For lunch it might be an option not to sit down and spent a lot of your valuable sightseeing time but instead to go to a bar and get some sandwiches (panini or tramazini) or a small hot dish like a pasta for a couple of Euro.
Below "middle of the road" and more on the outskirts, you can find some places who offer (often open only for lunch) simple, hearty, but fairly good fair mainly to the local customers for e.g. € 8 for cold appetizers, hot pasta, wine, water. If you happen to run into one of these, it might a worthwhile experience of the local life.