That's simple really. Just make a "Total Garage Stat's" page. It can be linked in the profile and over on the left menu under "Your Garage", or even under "Your Info".
Within, you can have total costs both as a grand total and a yearly, total mileage driven, average MPG/ 100l/K, etc. Both as a grand total and/or yearly totals, if people used tags, you can list them by most popular, etc. I use the tags to identify the gas station I buy the gas from for example.
Time I understand, having managed a few sites already. It can be absolutely crazy to keep up! And certainly you have to have a bread winner to keep sites such as these going! lol
But, if you guys ever need assistance, by all means ask.