Wall Street continues to draw new lines in the sand when it comes to Sirius XM Radio (Nasdaq: SIRI). Standard & Poor's analyst Tuna Amobi is the latest to revise his price target on the satellite radio giant higher. He is sticking to his buy rating, but bumping his near-term price goal from $2 to $2.50.

This isn't pocket change. Sirius XM has 6.5 billion shares outstanding now that Liberty Capital's (Nasdaq: LCAPA) 40% preferred share stake is in the profitable mix. Amobi's arguing that the company is worth $3.25 billion more than he thought before yesterday's conference call.

CEO Mel Karmazin's dangling the likelihood of an upcoming subscription rate increase was enough to send shares off to a fresh two-year high yesterday. Given the largely fixed overhead at Sirius XM, any additional subscription revenue will be like a Twinkie to a dieting worrywart -- it'll go right to the bottom. Sirius XM has enough in tax loss carryforwards to offset years of pretax profits. If Sirius is able to squeeze another $2 a month out of every subscriber, we're talking about nearly $500 million given its current base of 20.6 million accounts.

Testing its pricing elasticity naturally has its risks.

There are two arguments to consider. On the bearish side, there were nearly 6.4 million Sirius XM cancellations last year. A service can't be too cocky when nearly a third of its base deactivates their receivers in any given year.

On the bullish side, last year's defections were more than offset by the gross addition of nearly 7.8 million radio fans. Save for a two-quarter spell during the first half of 2009, Sirius has always found a way to attract more listeners than those leaving. It was also during this span of time that Sirius XM raised its rates for secondary receivers and began charging for Internet streaming, but the net cancellations were likely a recessionary reaction. The automotive industry was also in a funk, and that's certainly not the case now. Leading showroom operator AutoNation (NYSE: AN) may have lowered its vehicle sales outlook recently, but not enough to get in the way of the millions of quarterly car buyers who may be wooed by free satellite radio trial offers.

Getting to $2.50 won't be easy, but it's clearly possible if Sirius XM's inevitable rate boost is well received by its subscribers.

Is Sirius XM's market cap of $13 billion at a share price of $2, or more than $16 billion at $2.50, high or low? Share your thoughts in the comment box below.