Well, obviously it doesn't matter how expensive your violin is, or how good you are at your music genre -- it's about how popular the genre/tune is, and when busking, how likable you are (plus, putting on a good entertaining show, with emphasis on the "show" part, e.g. not subtle classical skills).
There's a list of all known Stradivarius instruments on Wikipedia and it's actually nice to see that quite a good number of them are made available to musicians via loan programs or specific bequests:
Stringed musical instruments also typically increase in value as they are routinely played. A trained musician, as I understand it, can readily identify an instrument that has lain idle.