you can get min and max price from any collection
For any collection.
$maxPrice = $porductCollection$productCollection->getMaxPrice();
$minPrice = $porductCollection$productCollection->getMinPrice();
Layer collection.Follow Follow Magento\Catalog\Model\Layer\Filter\AbstractFilter.php
$this->getLayer()->getProductCollection()->getMaxPrice();
$this->getLayer()->getProductCollection()->getMinPrice();