I'm a big believer in budgeting a little bigger on trips, so I don't have to worry about what I'm spending during trips. First of all, you need to budget in Euros. The USD (if that's your home currency) conversion to the Euro has really been fluctuating over the past couple of years ($1.30 - $1.50). That'll have a huge impact on what you end up actually spending.
Now I haven't been on this tour, but I have been to these cities, and they're pretty expensive. Europe tends to be a lot more expensive to what we're used to in the US. VAT, higher labor costs, costs of shipping everything into Venice, etc are built into the prices of everything. Also, Rome/Florence/Venice are hugely popular destinations and the locals know they can pretty much charge what they want. I remember on my first trip to Rome paying $20 euros for 2 slices of pizza and a coke, near a tourist sight. I've since gotten wiser
Budgeting will really depend on 1) how many meals on your own you have to pay for, 2) what do you want to eat (i.e. pizzerias vs sit down restaurants), 3) how willing you are to go out of your way for a restaurant (out of the way places are much cheaper) and 4) how much shopping do you want to do.
To me, 100-200 euros a day for 2 people is realistic (but perhaps a little low-depending on your shopping "needs"). You could do $100 per day for 2 people, but you're going to eat pizza and do very little shopping.