Your battery will run out daily before you run out of bandwidth.
Here's a rough estimate of uncompressed data with a dedicated program that only needs the stock data (like Bloomberg). The amount below would be far higher if you needed to visit a web page to get the stock quotes PLUS all the junk (like advertisements) on the webpage.
Basic streaming, all you want is ticker, last price, and tick (up or down)
Ticker 4 bytes
Price 5 bytes
Tick 1 byte
So one quote is 10 bytes
20 quotes is 200 bytes
Assuming you refresh quotes every second (unrealistic)
so now 200 bytes per second
12,000 bytes per minute
x 390 minutes when the market is open (9:30 to 4:00pm)
4,680,000 bytes per day
Totals to 93,600,000 bytes per month (~20 trading days)
which is 91,406 kilobytes per month
or 89 megabytes per month.
Most plans have a minimum of 500MB per month. Again this is highly unrealistic since people don't refresh quotes every second (and I highly doubt a 3GS is fast enough to handle it anyway). More like every other minute which cuts the bandwidth down dramatically. The 3GS battery will run out before you can complete one day of streaming every second.
I'd estimate though that you'd come nowhere close to hitting that bandwidth estimate streaming just stock quotes.