I've worked in two different gyms that gave away LM t-shirts at launches as door prizes, specific to the program launch. The people who ended up with them wear them religiously. The t-shirts aren't nearly as expensive as the rest of the gear. Other than that, I don't think participants go out of their way to get LM gear. Then again, in the US we have a poor selection of clothing, and LM isn't nearly as well-known as it is in say NZ!
Going out on a limb here because I don't know if this is allowed, but it would be pretty neat if your club could sell LM clothing. People are more likely to buy something when it's right in front of them, with no shipping charges.
Other fun ideas that would be even more affordable as door prizes would be those little arm sweatbands or other such accessories. And, clothing is often available on Ebay.
Good luck to your club!