I'm a person that is usually trying to save as much money as possible. If I draw my own book cover, is it okay? or do I really need to hire an artist?
I'd say, yes, of course, you can.
For instance, authors of children's books can many times do both the text and the artwork. And even stick figures can work...
It all depends on your artistic skill. (And the type of book, as the stick figures example shows...)
If you're working with a publishing house, they will have ideas and standards and may or may not want to use your cover. But then again, it wouldn't surprise me if artistically skilled authors have designed their own covers as well.
With a publisher, it's not just about art, but sales and PR as well... Or well, it's always about sales and PR as well... even if your cover is only visible on Amazon...
But, if you think you can do it, then why not?
Children's author Grace Lin drew some of her own covers and illustrations. J.R.R. Tolkien drew his own maps of Middle Earth. I'm sure there have been plenty of other authors in the past who have done this, so I'm sure you could do it as well.
If you're planning on self-publishing, you might have a better chance at using your drawing, since, as Erk said, traditional publishing houses may or may not use the drawing.
I personally think it's very impressive when an author draws their own cover/illustrations. It is cheaper than hiring someone, and if you believe you can do it, go for it!