Like guitars - or any other musical instrument for that matter - Ukuleles come in a variety of different price points. From basic plastic ukes that don’t cost much more than a take-away coffee, to artisan handcrafted instruments that cost thousands.
In terms of how much you should spend, it really boils down to the intent. For very young kids, it’s best to go for a basic $/£30 plastic uke and see if they take to it. Also, if they decide to throw it across the room or drop it, it won’t splinter.
For the more serious beginner player, then around the $/£100 mark is usually a safe bet. This price point allows you to get a well-crafted instrument and is more than serviceable to learn on. For the professional player, you’re looking at spending at least the $/£300 mark.