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From a sales and marketing point you need to establish your Budget for "customer acquisition cost" before you can decide whether the list cost is worth it!
Just get some assurance that the test list is an accurate reflection of the final list.
If the lists prove out to be "good" and you develop some kind of relationship with the broker, you can definitely negotiate pricing in the future (maybe even after the test).
If you have a proven service or product, many list owners will also consider working with you on a CPA basis or low CPM as an advance against CPA.