What is the reason or proper usage of "price" and "pricing"?

The two sentences are roughly equivalent, but not completely so.

Price means the amount of money one will charge to sell an item. "The price for this can of tomatoes is fifty-nine cents."

By extension, can mean what one has to give up to achieve something. "The price for success is losing your social life."

Pricing can mean the same thing (and can be used as a too-many-syllables version of the same word) but it can have the additional connotation of the strategy or system one uses to set prices. For example, deciding to hold a sale to coincide with a holiday, or to set loss leaders (to take a loss on some items to get customers into the store), or to offer free shipping on certain orders.

Tiered pricing is where a company offers different levels of service for different fees. ("Dropbox's pricing strategy includes a free, basic service level, in the hope that customers will use the service and like it, and then decide to pay more for additional storage.)

The wikipedia article on pricing covers several of the factors involved in pricing strategies and setting.

Alternately, "pricing" can be a verb meaning to apply or determine a price", as in "I'm using the label gun to price these cans of tomatoes", or maybe "I'm pricing the items for the garage sale" (where "pricing" means "to decide on a price").