Paris guiding runs from free to several hundred euros, and the tiers are genuinely different products rather than the same thing at different markups.
Tier one: tip-based walks
Daily, from several companies, covering the Marais, Montmartre, the Latin Quarter and the classic centre.
Guides are frequently excellent because their income depends entirely on the end of the walk — and many pay the company a fee per participant, meaning someone who gives nothing has cost the guide money.
€10–15 per person for a good two-hour walk is the working norm.
Limits: group sizes up to thirty, and they cover the famous ground.
Tier two: institutional
The best value in Paris and almost invisible, because it is not sold on booking platforms.
The Louvre runs staff-led tours at a fraction of private-guide prices, led by people who know the collection.
The Centre des Monuments Nationaux runs guided visits at the Conciergerie, Sainte-Chapelle, the Panthéon and elsewhere.
Several museums run thematic visits and workshops advertised only on their own sites.
City heritage walks and open-house weekends, when normally closed buildings admit visitors.
Finding these means going to the institution rather than a platform.
Tier three: small-group specialist
Where the price rises and can be justified.
Food tours in one neighbourhood — the Marais, Saint-Germain, the 11th. The value is not the food but learning to order for the rest of the trip.
Group size is the whole question here. Look for a stated maximum of eight or fewer. A group of twenty cannot enter a small fromagerie, so larger tours go to places built to absorb groups — the ones you could find yourself.
Thematic walks — the Revolution, occupied Paris, literary Paris, the covered passages — with a specialist guide.
Photography walks at dawn, which get you the city empty.
The Versailles question
Independently: RER C, timed ticket, audio guide included, and it works.
A tour adds transport and queue management on a genuinely busy day. Whether that is worth the difference depends on your tolerance for the security queue.
The gardens are free on most days and are enormous. The Trianon estate and Marie Antoinette's hamlet are separately ticketed and considerably quieter than the palace — for many visitors the better half.
What to skip
Half-day city bus tours. The metro covers the same ground for a few euros.
Seine dinner cruises. Expensive, with the food as the weak component. The Batobus or an ordinary river boat gives the same view.
Skip-the-line packages that resell a standard timed ticket at a premium. Check the official site first.
Practical
Book direct where the operator has a site — platforms take 20–30% commission.
Off-season, the same walks run in groups of six rather than twenty-five, at lower prices.
Licensed guides in France hold a national qualification, which matters for entry to some sites.