it really depends on...location, venue, and what you are planning to sell-
we have a couple pretty large craft shows around the holidays===that are quite costly to participate in---but the up side is they tend to have between 6000=10000 people go through- so chances are---you will make money-
i've seen others that only had a handful of people show up- and no one really made any money-
is it a show you have visited? could you possibly talk to a couple people who have done it in the past to find out if they get the customers & sales?
$50 is probably in the (going) price range- but how much will you have to sell in order to make any money?
before commiting you should really find out about the venue = try to find out how much traffic it gets- what kind of advertising do they do....and how many people are trying to sell the same things as you? sometimes if you are trying to compete with someone who has a following (they've done this show for a number of years and people go looking for them) you could find that you get passed by.
lots of research == but if you think it is worth it - go for it and give it a try- worst case scenerio is - your out $50 but you will learn alot.