How much should I pay my private tour guide? Is there an average?
First things first: there are official rates, but they are, to say the least, Italian, which to say Byzantine: €123 for up to three hours for groups up to €20 (which goes up if the guide has to speak another language or works nights after 8). Cut away the nonsense and you get an average of about €40 an hour — $50/60 — for licensed guides (some can be twice that, depending on experience and expertise). Daily fees also vary, usually between €300 and €450.
Pay attention to the word licensed, since plenty of freelance guides are not licensed and vary their prices downward to increase their volume. Many of the unlicensed are native English-speakers who are trying to make a living by giving guides on the sly, which is fine for you, less fine for them, if they're caught. Many licensed guides will also give you a daily or weekly rate, usually with discounts, with wiggle room to negotiate, since it's a buyer's market. Whatever you might read about a guide online, be sure to speak to him or to her beforehand, so you're comfortable with their language skills.
If you're planning an exceptionally long day or a particular kind of excursion, or one is recommended to you, be sure to establish the going rate beforehand. The Italian saying, "Clear understandings make for long friendships..." counts a lot in this tricky business, where the relationship between camaraderie and cash can sometimes produce awkward moments. Most are avoidable if you reach deals up front. That leaves you free to pay more, or to tip generously, if you're particularly satisfied.
As for so-called unofficial tour guides, the kind you may find lurking outside a monument or tourist venue, three words: use a book (or app).