Is this a fair deal?
Depends on what it's 50% of.
50% of $100 is a lot different than 50% of $10,000
As long as the fee covers your standard rate, then sure, it's 'fair' in the context of any other work you do.
UPDATE: Based on your comment...
I will do the design and take 500$ and they will take 500$
That may not be fair in the context of who's doing the work. Sounds like this is more of a sales commission type model. The big red flag I see here is it sounds like they're the ones 'selling' the project at a particular price point, but you're the one that actually has to do the work and, as such, you're really the only one that can estimate the true cost of the project.
As such, this could clearly be a risk, or a huge benefit. If this firm is capable of selling projects way over the estimated cost, you might come out ahead. On the other hand, if they sell every project at a low cost, you may end up losing money on each project once you take into account your expenses and time.