Thats a really good price, I paid over £2.50 for mine at my Pharmacists.
I dont know if they would be harmful to ketosis if overeaten, I have had a maxium of about 5 a day, but not really used them lately as the really bad breath seems to have subsided.
My chemist said they can be consumed freely cos they have no calories or carbs or summat. I got 12 packs from Ebay for about £7. then £3 delivery. I only have about 3 strips a week. I tell ppl to tell me if my breath smells but they dont so im assuming my breath dont smell or they are just being polite