Mopower.
Vinyl siding is sold by the square generally (100sq. feet ).
The price can vary quite a bit as there are so many different makes and qualities of siding . Do a web search on it and you'll get a pretty good idea . Mine will come from a local lumber company ( Lezzers ) as it'll match my house . You might want to figure Tyvek or simular house wrap into the price as well if you plan on heating your garage in the future.
It cuts down on air infiltration quite a bit .
If you haven't done vinyl before , you might want to see if Lowes , Home depot have one of those do it yourself classes available . It's not hard to do , but as with most things there are some do's and don'ts with it . I'm sure if you do a web search on vinyl siding you come across some tips on working with it . Not much of a direct answer . But I hope it helped a bit any way . All the best , John