That’s a factor that depends on your traffic, its quality and yes the PR on the site as well. I am not sure what the regular prices are, I can tell you what kind of offers I’ve gotten and why I have refused them.
The highest price for a link I’ve received on a PR2 site was £40.00 per month, but it’s usually around £20.00. They want the links on the homepage only, and there’s no way in hell I am passing PR juice to sites I barely know for such money.
And since they have contacted you and not the other way around, ask for whatever you feel comfortable. If it’s 200£ ask for that, and make them go away. If they want it that bad they will pay.