Some people swear blind that a drop of bleach in the water works to keep flowers looking fresh, although I would only do that for roses; for anything else I use a teaspoon of sugar or, if I have it, the packets of flower food you get with supermarket flowers.
I use them quite a bit for class, and a florist recently recommended I buy focal flowers (i.e. the main flowers in an arrangement) from a florist, the secondary flowers from a supermarket and the greenery from a garden.
The only thing I would say is that supermarket flowers do not tend to last as long as florist bought flowers, but when they’re cheap as chips, who cares?
Anyway, there are a few things I’ve noticed when you put together a bouquet:
If you want roses, but not the expense, you could try lisianthus (the deep purple flowers above) or ranunculas, which are pretty in a similar way but often without such a steep price tag (although they will never be as cheap as daffodils or carnations! )