Corn is a cheap grain, which is why mixes that include corn are usually cheaper than those that don't. That being said, corn is perfectly fine to feed to birds, so go ahead and indulge your pocket book!
For less than $15, I can get 50 pounds of cracked corn up at our feed mill. It's lots cheaper than sunflower and the birds love it in winter. It's a great source of carbs for them. I still use the more expensive stuff (like sunflower hearts and mealworms) because they offer a better protein source, but I put out less of those, which is nice for the budget.
The only caution is that corn molds easily if it lies on wet ground, so I stop using it in spring when things begin to thaw.