It's unbelievable that he bought a new car, isn't it?
Is it any wonder that he bought a new car?
Are they similar?
No, they're not.
Is it any wonder he bought a new car? = It's not surprising that he bought a new car.
His old car needed $4000 of repairs. Is it any wonder he bought a new car instead of spending that much money to repair one that was 15 years old? = It's not surprising that the did that.
If you say "It's unbelievable" you are expressing great surprise that he did so. When you consider that he has so much debt that he has to live with his parents to pay his bills, it's unbelievable that he bought a new car, isn't it? = I'm very surprised he did that.
I am sorry if my mild shock and disbelief misled you. I can see why it did.
I was more in "mild shock and disbelief" at the situation (in Barb's example, that his car was so dilapidated) than at the reaction (that he would buy another one). In my examples, I was more mildly shocked and incredulous at Icarus' stupid attempt at flying so close to the sun, than that the wax melted.
Barb's examples are much clearer and closer to the meaning of the phrase.