We don't recommend using this setting unless you have specific need for it, and only after you fully understand how it works.
By default we do NOT allow users to set a price higher than what is set for a particular item in your BigCommerce catalog.
The reason for this is that our system is designed to quote using the standard price or or a discounted price. BigCommerce does not allow increasing the value of the cart.
If you are using a quote template that does not utilize one of our magic checkout links, then it really doesn't matter what your product catalog has stored for a price. This is esp. true if you have hidden items, items set as not available to purchase, or items with a $0 price assigned to them.