Assuming that you are not spending beyond your means, I'd be inclined to bluntly explain how you can afford the things you do (e.g. putting away a little each paycheck to save up for big purchases, budgeting your income with certain portions earmarked for the types of purchases she comments on, simply making enough above your regular expenses to be able to afford to splurge occasionally, supplementing your income with investments, etc.).
If you're teaching your children specific financial lessons (if not, I find this blog to have some fantastic suggestions), share this with your MIL, and explain how your purchases are in keeping with the lessons you're teaching your children.
If she doesn't take the hint, though, and persists in making negative comments (entirely possible; some behavior, particularly criticism of children, can become so ingrained that it becomes nearly reflexive), you may have to make her uncomfortable with your responses before she'll be able to evaluate whether she needs to change her behavior. Its important that you do this without offending her (or your wife!) if you want to maintain a relationship.
To do this, you might try putting the implied criticism back on her.
Explain why the purchase wasn't actually a problem (as above), but then take it a step further, assume that she's merely projecting her own problems on to your family, and offer to teach her how to apply those same lessons to her own finances.
If she says something like "that TV must have been too expensive" or even "I can't believe you wasted so much money on that TV", you can reply with something like "it really wasn't that difficult since we planned ahead for our purchase, and saved up for it. I've been teaching our children about the value of setting aside a little bit of money as part of long-term planning for big purchases, and this is a concrete example for them. If you'd like, I could give you some pointers on how to manage your finances like this, too."
On the other hand, if you are at or close to the limit of what you can afford, it may be a bit difficult to present it this way (and you may have to give some serious thought to whether her criticisms have some merit). In this case, you may have to simply say "we've decided that this purchase was worthwhile; if you disagree, please find a different way to express it, as you're coming across as being very critical and it is upsetting."