Why does it matter how much he spent if it can't be returned. If it's so old it won't sync with your other devices I suggest it's a used computer and it's likely he purchased it from a private person. If it came in a sealed box and perports to be new than I would wonder if he bought it from a private person who acquired it illegally. I suggest the problem is more about how he purchased a seriously out of date computer and whether or not it can be returned.
if you want to keep the computer, then cost is not as relevant as where he purchased it is. Knowing whether or not he paid too much, unless he bought it from a reliable source, is not helpful. If he bought it from a store with a good reputation I suggest the price was reasonable. Then the question is do you want to keep it or not.
I suggest you trust that your husband made a reasonable choice as to cost.If whether or not you keep it depends on the cost, then tell him so and if the price is too much ask him if you can return or exchange it. Since you both decided to not buy a computer I ssuggest you use that decision as a way to start a conversation about cost and how it fits into that decision.
I, too, wonder about the compter's age. My computer is 14 years old and I can do everything I want to do on it. It syncs with my phone and tablet. I wonder if you just need help getting it to do what you want. A reason to ask where he bought it so you can ask for their help.
I am thinking he bought this computer because the cost fit better in your budget than a newer or more complete one that you'd decided against buying earlier. Therefore, because of the cost, you can't return it in exchange for a "better" computer. Whatever the reason for having gotten this one I urge you to just be honest with your husband about the difficulties you have with this one and work out what to do together.
I think I might've been disappointed that my husband made a purchase for something we'd decided together to not do. And I would've wanted to be involved in making the choice if you agreed that buying a computer was ok after all. I wonder if you're hesitant to discuss this with your husband because of feelings you don't know how to deal with.