Our club has participated in a few events locally here in Ottawa and I can share a few insights.
We've been to two "Import Night" shows (basically kids with hopped up import cars) and had a small outdoor track set-up and ran mock races every half hour. The exposure was nice and we had lots of spectators when the nitro guys fired up. The electrics didn't do much for us though and overall we didn't gain any new club members. Not too sure about the cost to the club, but I'll look into that and get back to you.
We've also done some mall shows that I believe were free. These were the same idea as the Import Night shows but we ran on the mall floor which isn't great for traction.
The last thing our club participated in was another show at our Science and Technology museum. Again just a demo with a small oval track and it was more of a show and tell session than anything else. This one was another freebie as we were asked to participate and it was over the Christmas holidays. We got the standard how much....how fast....can I try type of questions, but again membership didn't increase as a result.
Hope that helps.