That supposes "supporters" have disposable income, and don't support a candidate in other ways like donating time, or canvassing.
The disposable income can be as little as $1 (though you also need a credit card and access to a computer with Internet), so it's not too exclusionary.
You can also qualify by getting 1%+ support in 3+ "approved" polls.
>though you also need a credit card and access to a computer with Internet), so it's not too exclusionary