Unfortunately there are slim picking for cars just like your Mazda 6 wagon in the US today. Acura has there TSX wagon a few years ago but these are still above your price range. Same with Volvo wagons plus there might not pass the fuel economy and reliability desires. So Thinking out of the box a little.....
Mazda 5 - 2012 or newer
Assuming you don't need 3 across the back the Mazda 5 comes close to your desired. MPG is 28HWY / 21City (24mpg /combined). Most out there are sports trim with Automatic
Subaru Outback 2010+
The Outback is a great option and if often recommended for people on these pages. You can easily get a 4 cylinder 2010 or newer for under 10K, but you may need to up your mileage to more like 80k
Ford C-Max 2013
This may be the best option if the cargo space is big enough. Well equipped, good reliability (as long as you avoid the energi model) and 40mpg combined.