al, first off, Viatour is not a tour agency, they are a middleman agency that sells other peoples tours. You will likely do much better to book your own tours directly with the companies.. Some companies do have better or worse reps for the quality of guides, booking though a middle man you have no gurantee what you are getting( note they do not include the name of the tour company that you are buying a product from). Its best to decide which sites even need a tour , and try and do a few things on your own. For instance, Paris, a good daytrip is to Versailles . Admission to Versailles runs from 18-25 euros( if you go in summer on weekends there is a garden fountain show that makes the admission 25 euros, the rest of the time is 18 euros) . Taking the commuter train to Versailles on your own is easy, millions of tourists do, and total cost is 7 euros for a return trip.
So total cost for a visit to Versailles would be maximum of 32 euros, or a bit less, but most tours start at around 50 euros. Big savings Plus with doing it on your own you can spend and enjoy the full day there, not just the 4 hours most tours give you ( and that often includes commute time, so not much time at palace!) . I could give you more examples, but you see how it is. Some tours are worth the extra money , many people love taking walking tours, they are generally reasonably priced, you get alot of interesting information , and the groups are usually not massive. Anyways I would repost this question with a little more information, where and when are you going .