One of the appeals of the ukulele is that it is relatively cheap compared to other instruments. But, as always, for a quality ukulele, you'll need to spend a little more.
The very cheapest ukes available can be bought for less than $20. These ukes will usually be playable and most people start off with this sort of ukulele - I did. However, if you're serious about the ukulele, it's worth spending a little more.