I've found that, oddly enough, both the high-end and the low-end tend not to hassle you much, and instead it's sort of the middle-range optometry shops that are trying to make a bunch of money on in-house lens/frames sales.

Just about the cheapest place to get an eye exam is in the Wal-Mart vision department, and they're happy to hand over the prescription. Of course, they run the vision dept in part to try to sell you lenses/frames (or contacts), but it's a very corporate/streamlined place, and the actual employee working there doesn't mind if you want the prescription to take elsewhere.

On the other end of the scale, places that look more like a doctor's office than a frames showroom tend to be pretty professional about it. Since I have very strong myopia with some risk of retinal problems as a result, I've been going to a place that's a partnership between an optometrist and an ophthalmologist (M.D.) who do a more thorough eye-health exam, and there it's basically expected that I'll want to take the prescription with me.