I know you asked about annual costs - but those would be small potatoes compared to the cost of getting the vehicle into Uruguay in the first place. If you have the money to do that, you don't need to worry about annual costs!
You say you want to import your vehicle - do you mean, import for all time, or only to use it for some specified time while you are in Uruguay, as a tourist or on some type of visa?
It makes a difference. To import a vehicle it must be new - a used vehicle may not be imported at all unless you are a diplomat or it's a diplomatic service vehicle according to this site which is of the French Embassy in Montevideo. It also lists the percent taxes you must pay:
Assuming your vehicle is not being imported from within the Mercosur jurisdiction, you'll pay (at least) 23% customs tax, then 40% for a diesel or 60% for a gasoline vehicle, plus the IVA of 23%. Note the taxes may total more than 100% of the new value of your vehicle!
Here's another link which confirms non-citizens of Uruguay may not import any used vehicle except for certain exceptions outlined in the article:
If you wish to bring the vehicle in for temporary use while in Uruguay (not permanently, not importing) see this page to see the requirements for an extra-Mercosur vehicle for temporary use (scroll about halfway down the page):
I don't see the costs listed for temporary entrance but you could send an email to inquire, to
info@aduanas.gub.uy
You could also contact the Ministry of Tourism, a page for tourists is here along with their contact info: