Pretty good idea actually. If you want to get a brand (e.g. the SLP logo) promoted, then the way to go about it is to use the balloon/t-shirt combo in a populated area, handing them out to people.
Will take a good look at prices, and get back to you on cost-effectiveness compared to leaflets.
[UPDATE]: yup, it’s looking good. You can get 1000 specialised balloons for just £108 from these guys.
I’d call that good value for money – leaflets generally cost about the same, and balloons will be far more visible and dare I say it fun!