I'm using Metager - you pay a very small amount per search, the amount is defined by which indexes and options you configure to use. It's pretty cheap - I loaded up 10€ a few months ago and still have about 2/3 left, and it's adfree. There is also the option to use completely anonymous tokens to "pay" for your searches, which uses a little bit more of your balance, but can't be tied back to your account. Search settings are saved only locally in the history - using the browser extension prevents losing your settings when deleting browser data.