Timing belt jobs are always $400+
I agree that the extra charge is high because:
A. You have all of the accessory belts off, and there is no extra labor putting new ones on (belts are $10-$15 each, then factor in maybe $5 on top that the shop may charge per belt).
B. The water pump should be factored in already into the timing belt job, as so should the tensioner. A timing belt job is not complete without replacing each of these components.
C. I dont know what your mechanic means by all of the seals, but the only ones that should be replaced are the valve cover gasket and tube seals. Nothing else should need to be changed.
So if you were to look at it, about $80 extra for the seals and belts, with no extra labor because everything has to be put back together to be put back on. Water pump is accessible because the belt is off, as well as the tensioner (and should have been included in the initial price), so I dont see why your mech. would decide to charge $300 extra. $120 at the most...