If you don't mind me asking, how much of a markup are you adding for those shirts? You're right, $27.99 is incredibly high.
I've done t-shirts through spreadshirt.com. A high-quality shirt with a full-color image has a base price of $14.40, so even with a $5 markup you're keeping it under twenty bucks. Or go for the lower-grade shirt that's four bucks cheaper.
(Hm, can I get a referral credit... hmph, not yet. Fine, take this advice for free. )
As for their importance... depends if you're looking at them as a revenue generator, promotional tool, or a little of both.
As a revenue generator, you may want to look for something a little more "general wear" than a Street logo. There are a number of webcomics which offer shirts which may be "in-jokes" for fans of the book, but to the general public they're just a normal shirt. For example, take a look at the t-shirts offered by the creator of Questionable Content. A number of those designs are clever on their own, but inspired by the strip itself. I'm not sure if Street offers anything like that, but it's worth considering if your goal is to sell lots of shirts.
If you're looking at it as a promotional tool, then you'll want to work with the logo but you might have to consider a) asking for less of a commission and/or b) accepting lower sales.
Of course, there's also the personal satisfaction of seeing people --- particularly cute members of your preferred gender -- with your logo on their chest.
Me, I'm planning to offer shirts when Midnight Cross launches in August. I'm keeping the commissions reasonable -- $5 seems plenty -- and I'm tying it to a Premium Content program where each item purchased gets you one free month of Premium access. For now, I'm sticking with a basic logo design, but I'll be watching how readers react to things in the story -- if something seems to grab hold of the audience particularly strong, then it's worth coming up with an appropriate design.
I also had a few made up for myself, my wife, and my three main cheerleaders. See aforementioned comment regarding cute members of the preferred gender.