$300 buys a lot of gas or train rides. You could even buy a new pair of walking shoes, and get out the door to talk to customers face-to-face. It's scary out there, but actually speaking to people to get signups is cheap/free, and something really only a startup can do since you're not going to get huge numbers from it.
I'm struggling to find a single startup concept where you couldn't be served by walking through your city and finding a potential customer. If one exists, they're probably targeting the wrong customers.
There's an old idea that might work: Get a lot of business cards; always carry a stack; spread them around, e.g., in any retail location, leave a few here and there, maybe on the counter top near a cash register, maybe just drop some on the floor,
maybe at a sporting event, concert, political meeting, any meeting with a lot of people, get high up and give a big side arm throw and let them flutter down. Sure, then quickly duck, hide, and get away before get accused of littering!
Or, gee, for the $300, hire some kids. Each kid gets a stack that has a promo code unique to them, and on your Web site landing page ask for the promo code and, thus, find out which kids are doing the best work! Sure, use different promo codes for your own efforts at such advertising -- basketball, football, politics, rock concerts. Uh, don't necessarily have to pay big bucks for NBA or NFL tickets; how about a college or high school sporting event?
All or nearly all of the above are from a book or article I read long ago about a guy who was trying to sell cars. He was just a salesman at a dealership run by someone else, but he wanted people to come to the dealership to see him by name, in person, as the guy with uniquely good car deals, etc. With those business cards and more such outrageous stuff, he claimed he got a lot of traffic and sold a lot of cars.
For the business cards, sure, cheat some on just what the heck such a card is and, thus, make them also have enough information, if only a tease, to get people to get to a Web landing page or some such. So, they look like business cards but, of course, are really hand bill advertising. Maybe such hand bills are illegal but just accidentally losing a stack of business cards is not?
There are many more possible gutsy, outrageous, non-standard, maybe crude, vulgar, socially awkward, embarrassing, borderline legal, original, creative, never seen before, etc. marketing approaches.
One description of such stuff is guerilla marketing.
I think the car salesman you read about is the legendary Joe Girard who once won the Guinness World's Best Salesman title. He was also mentioned more than once in Cialdini's "Influence".
I think it depends a lot on your product how effective this will be. I did a lot of this while EnvKey[1] was in beta, setting up in person interviews/meetings via my network, and while I did learn a lot from it, it didn't lead to getting many users. This was primarily an issue of timing: EnvKey is a tool for managing configuration and secrets, so the most natural time to adopt it is when you're starting a new project. It has an import feature, but convincing someone to import an existing project is just a much higher bar than convincing them to try it on something new.
The problem is: what percentage of companies happen to be starting a new project now or even in the next few weeks? There are plenty of them out there, but the odds of getting someone at the right moment when reaching out one by one in meatspace are too low to make it work. In this situation, I've found that a hybrid approach works better: use organic marketing, meetups, and conferences to cast a net that is sufficiently wide that at least some of the people that see it and like the concept are in a position to adopt it right away, then keep in touch to find out how it's going and what the blockers are. This has been effective for getting our first batch of happy production users.
Yeah, just walking into a supermarket and expecting to find a B2B connection is unlikely, but like you said, walking into a convention where your likely customers are is a great opportunity. Even coupling that with the other guys suggestion of just taping fliers everywhere outside the convention or dropping business cards on tables in the entryway lets you skip buying tickets to the convention but still get some boost from it.
$300 will get you pretty damn far that way, enough to get a customer or two that you will hopefully be able to provide good enough first-class service that they'll help you from there.
I'm struggling to find a single startup concept where you couldn't be served by walking through your city and finding a potential customer. If one exists, they're probably targeting the wrong customers.