I paid approx $2.25 one way to downtown. On the plus side, if there are traffic issues on either I-5 or I-405, you'll save time.
Unless you live right on the light rail line, you may have to employ a second mode of transport (no parking at the actual light rail stations!) which kinda defeats the positive environmental impact. Currently, ridership hasn't matched up to expectation, but that may change over time.