I wouldn't use pellets alone unless your hamster has a health problem that makes them have to be fed mashed food only.It just isn't appropriate to hamster's natural behaviors.
In the UK Harry Hamster is just as good if not better than Science Selective for most Syrians,and it's still fairly affordable overall
Hamsters generally don't overeat like other animals do,so there is no reason why putting more food than can be eaten in a day is necessarily bad.That only really applies to fresh foods which will spoil.