Like Nick, when I'm just going out-and-about in the neighborhood with the camera and one lens, I leave the lens cap at home. I usually carry only my house keys and a small pouch in which I have a fresh battery and a second SD card if needed. Usually I check the battery before setting out, but after one day neglecting to do that and being halfway around the lake when the battery decided to die on me, necessitating an immediate walk back home, I quickly learned my lesson and always carry a spare battery with me now. The pouch with the battery and card slips very neatly into one of my pants or shorts pockets, or in colder weather, a vest with pockets or a jacket pocket. When going out with more than one lens I usually have a camera bag with me and am wearing a pair of pants or shorts and (if needed) a jacket or sometimes my old Domke photography vest. If switching lenses in the field, both lens caps for the lens about to be used go right into the camera bag.
While out shooting, whether it's just walking around the lake or at a park or place I've gone for the afternoon, I take the lens cap off before beginning to walk around and then don't put it back on until I'm done shooting for the day or when swapping lenses if using more than one out somewhere.
One of the first things I did when buying my new gear was to order extra body caps and rear lens caps from B&H, providing peace of mind in case of loss of one of those vital items. If I ever lost a front lens cap, I would simply order a replacement in the appropriate size, but in the meantime the lens would still be somewhat protected as I always use the lens hood and I have clear filters on the lenses as well.
It's a good thing it's probably going to be limited to about $10; not much to risk.
