Curriculum comes in all shapes and sizes. I've used complete kits (all subjects, one purchase, super easy), and I've selected subject by subject. Currently, I use a complete kit for my history and humanities studies with individually selected curriculum for math and science. Then we supplement with outside lessons.
Deciding which specific curriculum to go with for me was a matter of experience. I was a classroom teacher for a couple of years, so I was able to get some hands on experience with a few choices there. Then it was a lot of reading and research and trying out things with my own kids. I've had great luck with some and still use my first choice, and others have been an utter failure that I've had to toss mid-year.
Cost can range from free to very expensive. I tend to lean toward pricier options that last are designed for long term use, so in the long run it works out to be very economical. The biggest thing to realize is that you do NOT have to break the bank to educate your child.
This year I bought a bulk of our curriculum from Mardel. Other places I've purchased from include Christian Book Distributors, eBay, Craigslist, our local homeschool group, and a local Facebook buying/selling page. I also greatly rely on our local public library for a large number of our trade books.
So, stay tuned for our 2014-2015 curriculum choices!