Item value is determined by:
1) the cost of said item, if purchased at a store (non-cursed items)
or
2) the amount you would have to pay - based on the market rate in your area - to banish / exorcise / permanently remove the item from your life (cursed items).
Also, with Monkey’s Paw parties, there is no swapping. Once you touch an item, it is yours permanently, unless and until any magical prerequisites are fulfilled.