It is expensive. But when I first started doing LFs I didn't want to take any chances with anything. So I had a pre-colonic and two post-ones (a few days apart) with each flush.
I still do the two post-flush ones, since it's amazing what gunk still comes out of me.
I think it's a good idea to have the passages nice and clean. After several flushes you'll find you only have to do them once or twice a year and cost won't be such an issue.