You must live in some neighborhood, if they have calligraphy contests, especially if said contests are serious enough that they ban things like guidelines.
That said, why would guidelines not be allowed, but lightboxs are?
My thoughts exactly!
I would say my Lightbox was one of the things I ended up using way more than I though. At first it was something I did not think as necessary, pencil lines and then erase was good enough for me. But my boyfriend found an offer and purchased one for me. And it was a game changer! I now use it for almost everything. I just have a set of guidelines in different sizes printed and use them with my Lightbox for even a simple thank you note. My mother and sister keep asking me for "pretty birthday notes" to accompany their gifts to friends and the most I have accomplished for the "please ask me with enough time" is like 2 hours in advanced. So most of the times, I take out my Lightbox, write them a quick card in some free minutes of my time, let it out to dry and forget about it. Then they just take it just before leaving for the gift-giving, already dry and nothing else for me to do! For me, it makes calligraphy really easy and fast!
That said, I would say also that if you are really sure you would only use it for that contest and then it is going to get forgotten and not used, maybe it is not worth it. $20 is not much, but it can be better spent in something more useful for you.
And good luck for that contest!!!