I also have an astigmatism and have one eye weaker than the other. I'd never recommend only wearing the lenses in the weaker eye as it means the stonger eye is going to strain more, resulting in that eye becoming weaker as a result.
I pay for a 3 month supply and it usually lasts me about a year, maybe even more. I don't work at the moment and I'm only in uni two days a week so I don't wear them all that much, I tend to wear glasses around the house. If it's any consolation mine cost just over £100 for a 3 month supply.