Probably the hardest part of prepping for us has been find the cash to get extra stuff to store and have for emergencies. Also some emergency preparedness stuff can be expensive. In fact a LOT of it has been going up in price as more people worry about the economy, natural disasters and weather changes and want to be prepared for the worst. So for those of us whose income has gone down and not up with the economic problems being prepared can be a real challenge.
On the the things we have done is to switch out the kids of things we buy for birthdays/Christmas etc. We ask for preparedness gear, or weapons, or _fill in the blank_. We then are able to make double use of the money we would spend on birthday gifts anyway. Sales and coupons are a big deal as well. I regularly check for sales on things we use a lot, for us it is some weird stuff like coconut flour and oil, but also regular things like beef for jerky, Ziploc baggies, and even a real steal we found at target on hand towels and wash cloths.
The other thing we do is to try and buy stuff with our regular grocery budget money and stretch it to include extra food for storage. Kroger has a really nice system of sending you coupons for things you buy regularly and I make sure to use these before they expire. Granted most coupons are useless to us for food anyway - with a gluten free household you find that almost everything they give coupons for contain gluten still I do check for the occasional sale and coupons for things like nuts and coffee, cheese and organic produce that we try and eat.
I also do a lot of research and we watch TV shows to find ways to be more self sufficient and prepared - all of which is free. Most importantly we are learning to be better stewards of our money which to me is probably the most important "prepping" for our family.