We have done many wedding fairs, roughly 4 to 6 of them per year. In my experience, the smaller fairs are often more worthwhile. As a matter of fact, we have dropped a major wedding fair this year because the return wasn't anywhere near as big as the smaller fairs. I suspect this is because with the larger fairs there may be many exhibitors who provide the same service and brides are just running around collecting as much info as they can. At the smaller venues, you often have a chance to actually speak with brides, which makes it far more likely that they will book with you.
Our success rate also depends greatly on how well our booth looks and how well we are prepared. We have pop up roller sign and dress our table very attractively. We also make sure we have plenty of literature which lists our services and prices. We have put together a special bridal package leaflet as well, so our marketing is more direct. You will also find you are far more successful if you stay standing, preferably in front of or to the side of your table versus behind it. The idea is to make it easy for them to interact with you.
At each bridal fair we always do a free prize draw. One bride who attends the event wins a bridal package. It's a great way of building your mailing list. We draw the name at the end of the event and phone the winner. We mail everyone else (by post or email) to let them know they didn't win, but include a special offer instead. We get lots of new salon clients, not just for bridal treatments, but for long-term treatments. The free prize draw has increased our return hugely, so I firmly recommend it. Have plenty of forms and pens handy, as brides will crowd around your table to get their chance to win. We ask for name, telephone numbers, post and email address, wedding date and permission to mail to them.
We also often get our table free by offering free services for the event's models. This also always gets our name mentioned, so everyone hears about us. Most events will even direct brides to our table to let them know they should sign up for our prize draw.
By the way, when determining your cost of the event, don't forget to determine if you are losing any client time. If you could have booked £200 worth of services that day, your real cost will be £430, not £230. There might also be other incidentals, like travel time and petrol, food, etc that you wouldn't normally have on a regular business day.
Hope that helps!