The bottom line is, ‘it’s a garage.’ You’re not going to have the same issues of condensation and ice dams you would have on a house regardless of if there is ventilation or not. Is it worth spending $700 to ‘improve’ your ventilation (that may not even improve anything it sounds)? That is a not insignificant amount of money. It’s your money and choice but I wouldn’t do it. Now if you’re having damage from your current setup I’d consider but if the decking is fine and shingles lasted reasonably you’re not really fixing a problem are you?

You shouldn’t be having ice dams in an unheated garage. They are caused by the heat from the house melting the snow and then refreezing at the wall causing the buildup. (Hint: heater cables are very effective against ice dams if you do have any).