But surely, as said, it depends.... on what stuff you have, on what space you have, on what you enjoy using, on why - for what purpose - you are making the card, on what you know sells...?
Decoupage sells. Flowery, ribbony things sell. Watercolours sell. If I am making cards to sell for either a charity or a Scout Group's funds, then I go with that. Then when I have a need to stitch I do a load of those. Then my nails break..... But I usually stitch or cut bits while 'watching' TV. You can't mess about with ink and embossing then so I rarely stamp these days. It's not been my thing for a long while.
I cannot - will not - afford some of the stuff either; you seem to need so many bits @ £5+ each that it's ridiculous. (I still can't get over those 38 special pens @£2 to colour one picture.) And then add up the range of distress inks for example.......and its not for me. But if I am making a card for a special purpose then I source the best I can and use up any remains later.
Martha, I rather fancy you best enjoy playing, experimenting, using variety as the spirit guides you. I love the results I have seen over the years......Don't let SU stop you!!