This is like buying a used car, pricing is negotiable. Cores are hard to sell and there are lots of sellers and consultants.
Write a detailed proposal telling exactly what your are doing and what your willing to pay. In your case the value to you is about $10 * 300 * 3 years = $9000. So maybe offer $4500. Request a compiled sim model and testbench to evaluate the core.
Email it out, and you should get some responses. Deal directly the author or his company rather than resellers.
If they don't respond, or can't provide a model and testbench, cross them off the list. Like you say, you can always call NetChip instead.
-- Mike Treseler