I never get eye test at asda opticians. But i saw their sites signs that an eye test at Asda costs $15. Besides, they also provide free NHS eye test for some people. In details, If you are Scotland, you can get free NHS eye test. Besides, people over 60 years old and children under 16 can also get free eye test. What's more, they also provide free eye test for low income people. Compared other places, it is cheap. You can have a try.
I had an eye test two years ago at asda opticians for free because I took part in the discount activity in which I bought a pair of eyeglasses. About the price now, I suggest you to telephone their store and ask it before. And their service is good. I think it is worthy for you to have the eye test there.