Using php constructor to initiate API class

I was wondering how to use the constructor to initiate the API class I use in the rest of the class. I need it to be self contained to use in separate a controller.

class Coinbase
{
    private $configuration;
    private $client;

    public function __construct($apiKey, $apiSecret)
    {
        $this->configuration = Configuration::apiKey($apiKey, $apiSecret);
        $this->client = Client::create($this->configuration);
    }

    public static function getBitcoinPrice()
    {
        $buyPrice = $this->client->getBuyPrice('BTC-EUR');
        return var_dump($buyPrice);
    }
}

In my controller I call my own Coinbase class like this:

    $coinbase = new Coinbase;
    return $coinbase->getBitcoinPrice();

This gives the following error:

Using $this when not in object context in Coinbase.php (line 23)

What is a good way to solve this and why is $this->client not configured in the function getBitcoinPrice().

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