Part of the reason smaller contracts are (relatively) cheaper is that the closing and transfer costs are more or less fixed; on a per-point basis, that matters especially for contracts that have a lower market value (i.e. a higher than average $/K ratio). So, if the market decides that a particular $/K ratio is worth X per point, the transfer costs on smaller contracts eat a larger share of X.
The other thing to consider is when you are disposing of them, an owner needs about 280K points to minimize the per-point program fee. Owning a total less than that means the program fee is proportionally more expensive. So, that further marginally diminishes the value of a small contract. (It is only marginal because someone who is adding on won't care as much.)
Don't worry too much about an extra thousand dollars here or there in buying the contract. That will very quickly be lost in the noise in overall costs to own.
Finally, if you are very close to a 70K multiple, it may be worth getting just slightly over it to get one extra HK credit. For example, 420K might be worth doing vs. 400K. Those are $159 per if you have to buy them, and that's more likely if you take shorter trips in lower seasons and smaller units. If you are buying e.g. CWA, the extra 20K points cost marginally less in fees than buying an extra HK credit would.
