Just picking up on a few points....
- Tie downs are invariably cheaper than hangarage, perhaps a little, perhaps a quarter the price. Some aeroplanes (anything fabric covered or with significant wood in the structure for example) I'd avoid tie-downs absolutely. For an all metal flying school workhorse, it'll need cleaning more often, de-icing in the winter, and a good set of covers - but tying down is relatively harmless.
- Sharing an aeroplane (buying into an existing syndicate more likely) works for a great many of us.
- Even if you were dead set on "going pro" my strong recommendation would be to do a PPL first then re-assess regardless. Until you have done that, or something very similar, you just don't know if you have the motivation and aptitude - or frankly, enjoy it enough.
- The PA28 is a good enough baseline for your thinking. For purely recreational flying you can probably pay a third its overall running costs. There isn't really an upper limit on what you can spend on hobby flying.
G