As others said, HRV is the way to go. They run from around $800 to $1500 or so, depending on how many electronic gadgets one adds and the size of the units and an installer is required. Make sure you get the proper cfm for your situation. They even have larger two core units and ones that come with de-humidification but dont know what they would cost.
In the summer, central air usually keeps humidity levels in check.
Home Depot has some good HRV units. I like Lifebreath myself.
http://www.lifebreath.com/main.htm