£10 does seem a little steep, although you do of course get a bus/tram day pass as well, which includes the Line 20 out to the zoo and Marton Mere, a popular route for tourists.
I am sure the bosses will be hoping that tour revenues will make up for any deficits incurred by the reduction in normal tram fares - the £3.50 off-bus day ticket will also be valid for the trams as well now. Previously, it was £6 for a TravelCard covering trams and buses.
I was also a bit skeptical when they made the CitySightSeeing service separate from the network last year and premium tour prices were introduced, replacing the 'normal fares and passes' that were originally charged. I don't think they are planning on changing this, however I wonder if the CitySightSeeing tour franchise service will suffer with more people choosing the tram heritage tour instead. In any case, it's BT competing with themselves again!