Google Play Developer Distribution Agreement
3.2 The price you set for Products will determine the amount of payment you will receive. A Transaction Fee, as defined below, will be charged on the sales price and apportioned to the Payment Processor and, if one exists, the Authorized Carrier. The remainder (sales price less Transaction Fee) will be remitted to you. The "Transaction Fee" is set forth here and may be revised by Google from time to time. Developer is responsible for determining if a Product is taxable and the applicable tax rate for the Payment Processor to collect for each taxing jurisdiction where Products are sold. Developer is responsible for remitting taxes to the appropriate taxing authority.
3.3 You may also choose to distribute Products for free. If the Product is free, you will not be charged a Transaction Fee. You may not collect future charges from users for copies of the Products that those users were initially allowed to download for free. This is not intended to prevent distribution of free trial versions of the Product with an "upsell" option to obtain the full version of the Product: Such free trials for Products are encouraged. However, if you want to collect fees after the free trial expires, you must collect all fees for the full version of the Product through the Payment Processor on the Market. In this Agreement, "free" means there are no charges or fees of any kind for use of the Product. All fees received by Developers for Products distributed via the Market must be processed by the Market's Payment Processor.