Adam,
Are you considering paying a referral fee for the introduction only, or just if you are able to secure the new account? Personally, I would only pay a referral fee should business develop from the introduction, i.e.- pay a % of the first order's net.
I have helped introduce various parties with the potential for creating a deal, with the understanding that I have something to gain should "a deal get done." I would not pay a referral fee just for an introduction, though.
As to the amount, I believe this depends upon your product, costs of development, your margins, etc. Perhaps you can view the referral fee as your "marketing" cost; so whatever you were going to allocate for marketing to secure this account, that could be your fee. Hope this helps, good luck!
Sean