Instead of paying for the list, if the 10k sample comes back low, the list may (or may not!) be low quality.
If you make some money, under sell itand get the remaining list for cheap ... if every email you sent made you $0.005, tell him it made you $0.001, then you'll get a cheaper list from him.
Alternatively, don't pay for the list and give him a 10% cut of any profits. From your sample you'll be able to estimate this value =]