We didn't actually purchase a writing program. Instead I set up private blogs. The kids would write bout things they learned or did, like a journal. Before the 'article' was posted they would have to edit for proper grammar, references, etc. once a week I give them a writing assignment - like doing a book report, research assignment, etc.
for lessons in grammar, we use the free website - http://www.chompchomp.com/
Have you looked at home2teach?
Both websites seem cheaper than a tutor. Though personally, I would either use a curriculum or tutor before using an online class for writing.
I'm kinda like kickbuttmama on this one though. I am too frugal. I would be hunting down and getting creative to implement a plan that was cheaper.