[650D / t4i] beginner would like a new, light lens for one-month trip

The simple answer is that there's isn't a good wide-angle zoom lens at anywhere near your price requirements. Because that's so, I suggest that you just stick with what you have, the 18-55. Shot at f/5.6, it's quite OK. If you want wider, set the exposure to manual (after determining the exposure you want), turn off auto focus (after using it to focus), then shoot overlapping photos and stitch them later (with any number of programs) into a panorama. You can't beat that lens for lightness