IDG Answers is a community of experts who are passionate about technology. Ask a question or answer one below.
The Jawbone is good, and you don't have to play $200 for it. I just looked online and found it for $170 new. That's still not cheap, but $30 is $30. You didn't mention which iPhone you have, but if it is an iPhone 5 don't accidentally order something that uses the 30-pin dock from the earlier iPhones. Most of the time you can use the headphone jack or Bluetooth for sound output, but there are a lot of speaker systems that use a dock, so keep that in mind.
You might want to consider something much less expensive that I've found works well, those little iHome speakers that pop up. It is not going to be able to match the Jawbone in either quality of sound or volume, but it is surprisingly good and you can find them for under $30. Small and easy to carry around too. iHome also makes some really nice sounding speakers that cost a little more, including one that looks like a little cube.
I picked up a Creative D100 for about $60, and it is the best unit I've found for the money. It's a Bluetooth, so you don't have to worry about the connector. It uses AA batteries, so you should include the cost of some rechargeables in the purchase price unless you want to feed it money every now and then.
If you want something really good and are willing to pay for it, the Altec Lansing InAir 5000 sounds awesome. An audiophile friend of mine has one, and it's hard to believe that a single speaker can put out sound the way it does. Downside is they list for $500 (but you can find them cheaper).