Magento – Magento2 : How to make a redirect with params

If your controller extend Magento\Framework\App\Action\Action you should follow rules of creation of the controller. So you should implement dependency injection of the parent class constructor (Magento\Framework\App\Action\Action):

/**
 * @param Context $context
 */
public function __construct(Context $context)
{
    parent::__construct($context);
    $this->_objectManager = $context->getObjectManager();
    $this->_eventManager = $context->getEventManager();
    $this->_url = $context->getUrl();
    $this->_actionFlag = $context->getActionFlag();
    $this->_redirect = $context->getRedirect();
    $this->_view = $context->getView();
    $this->messageManager = $context->getMessageManager();
}

Your construct method should look like this after a modifications:

/**
 * @param Context $context
 * @param UrlInterface $urlBuilder
 */
public function __construct(
    Context $context,
    UrlInterface $urlBuilder
) {
    parent::__construct($context);
    $this->urlBuilder = $urlBuilder;
}

NOTE: do not forget to remove old var/generation directory after implementing of modifications, because there are stored the generated files of controllers.

I couldn't reproduce your error, but I got an other one.

Then I did some minor adjustments to your code and it worked.
First drop the InstallSchema.php file. It does nothing so it is useless.

Then make sure that your plugin file is in Amaj/CorePlugin/Model/Currency/Plugin.php and it looks like this:

<?php

namespace Amaj\CorePlugin\Model\Currency;

class Plugin
{
    protected $logger;

    public function __construct(\Psr\Log\LoggerInterface $logger) {
        $this->logger = $logger;
    }

    /**
     * @param $subject
     * @param \Closure $proceed
     * @param $price
     * @param array $options
     * @return string
     */
    public function aroundFormatTxt(
        $subject,
        \Closure $proceed,
        $price,
        $options = []
    )
    {
        $this->logger->info('Amaj aroundFormatTxt');
        return '2';
    }

}

you don't need parent::__construct(); in the constructor because there is no parent class.
And I replaced $logger->info with $this->logger->info because there is no $logger variable.