Not sure of the size of your garage, but most of the garages I do, I only put one downspout on each side. THe going rate here in Northern IL for me is about $3.25 a ft installed. The bigger downspouts runs about $3.50 a ft installed. As for the leaf guard, there are a million different products out there. I personally only use a fine mesh screen that is made out of galvanized steel and powdercoated black. That usually runs about $2.50-$3.00 a ft installed. If you had ice and water shield put on under your shingles your screwed cause the screen needs to tuck under the first row.
My guess-timation with 2 downspouts per side is $430 for seamless gutters and downspouts. Extra. $175 for the scfreens.
Been in the seamless gutter buisness for 20 years.