We started homeschooling one of our children in middle school, and have continued. How your home school looks is highly individual. It will depend on the needs of your child and your family, as well as the resources available in your community. For secondary-level coursework (i.e., high school transcript credits and above), state regs make a huge difference. In some states, a home school transcript will be adequate for meeting state uni entrance requirements. In others, it will be much more challenging to meet those requirements without some external validation, such as standardized testing, accredited online coursework, etc. The state regs are definitely where you need to start, as well as any homeschool organizations in your area.
Until we need to obtain high school transcript credit, we have been fairly relaxed about curricula. All of our children are on somewhat freerunning plans, at their respective instructional levels by individual subject areas, with curricula selected based on the child and the subject. We've also abandoned curricula mid-way, when it became clear that it wasn't suited to this particular child, and modify curricula as needed. In particular, I generally de-couple basic skills from reasoning, use accommodations and technology to support both 2e and asynchrony, and try to avoid getting bogged down by busy work. For example, there is value in achieving mastery and automaticity with certain basic skills (math facts, handwriting, spelling, reading), but lack of fluency in those areas shouldn't prevent a child from progressing in their reasoning and comprehension skills, if that is what they are capable of. So I have them work on any lagging basic skills in shorter, focused skill sessions, and accommodate for those delays when the instructional objective is a higher-level reasoning task. Practically speaking, this looks like a lot of oral or scribed work, using speech-to-text/text-to-speech for certain children, and modifying English assignments so that, while the literature selections are at reading level, the written responses can be at a different writing level, if needed.
I also tend to steer away from comprehensive packaged curricula, mainly because I find them too rigid. Each child has areas that are stronger than others, so I would end up having to buy multiple levels for a single child, which seems like a waste of resources.
If you have a strong and motivated reader, one approach may be to set your child loose on a a great books reading list, for ELA and history. Assign daily or weekly reading response journals, paragraphs, or essays. Or assign an oral response essay (i.e., have them tell you about what they read that day, and their thoughts or feelings on it). I know of families who have their child write a daily email to a family member (working parent, grandparent, uncle/aunt) about their day's reading.
Cost is also highly individual. The most expensive is probably if you buy a comprehensive curriculum, especially as you may need more than one per school year, either because of pacing or differing instructional levels for content areas. Depending on how you handle ELA and History/SS, you could potentially pay only for stationery and office supplies, as many good reading lists are available for free online, and the great books lists are almost always available through your local library, or interlibrary loan. (Plus many are free ebooks on gutenberg or similar projects.)
There are a couple of free or low-cost options for mathematics. You can get a fair amount out of Khan Academy, there are OCW courses, and there are even a few sites like this:
http://teachers.henrico.k12.va.us/math/hcpsalgebra1/modules.html which includes everything except for the textbook for a complete algebra 1 class (there are a few other courses on the parent website). Eureka Math is a complete, research-based, K-12 math curriculum of free pdfs:
http://greatminds.net/maps/math/home. No textbook.
Science classes require a little more work, because of the lab materials. We've done book work at home, and labs through homeschool coops. We've also contemplated attending single courses at local public or private schools (both options are available to us where we live; you will have to investigate your area), at no cost in the public, and for a course fee at the private.
That being said, I do actually buy curricula. At your child's age, it's been literature-based history/ss (integrating ELA), math, and science. As my oldest was not in high school at the beginning of homeschooling, some of those curriculum costs (except for a small amount for consumables) are amortized over the rest of the family. I probably spent about $500-600 on my oldest this past year, but that includes the cost of switching history curricula midway, and curricula for two courses in science, including one lab kit.
