In my opinion, $1000 is way too cheap. Consider the fact you'll never be able to sell that file again. How much money will it make in a lifetime? Then at least double or triple that amount.
Also consider why the client would like the rights. They'll probably use it for a marketing campaign or several media publications for a long time. That's worth something.
I once sold an image (through another agency) for more than $5000. I wasn't involved in the negotiations but somehow the client (a big company) agreed to it. So my advice is, never go too low out of fear. You might be missing out on big bucks.