It depends on how much you value the garage space, how much the landlord wants a tenant, and how good your negotiating skills are.
If you're talking about an entire house--real rough ballpark number if other properties with garages are renting for $800 and the one property doesn't have it available--maybe in the range of $50-$150. It's all negotiable, though, so ask for as big a reduction as you want. Again, it'll really depend on how much you need the space (what you'll have to do if you don't get it), how eager the landlord is to rent the property, and your negotiating skills.
Hope that helps.