<event name="checkout_cart_add_product_complete">
    <observer name="customprice" instance="NsModuleObserverCustomPrice" />
</event>
<event name="checkout_cart_update_item_complete">
    <observer name="custompriceupdate" instance="NsModuleObserverCustomPrice" />
</event>
<event name="sales_quote_item_qty_set_after">
    <observer name="custompriceupdateqty" instance="NsModuleObserverCustomPrice" />
</event>

public function execute(MagentoFrameworkEventObserver $observer)
{
    $cart = $this->cart;
    $quote = $cart->getQuote();
    $items = $cart->getQuote()->getAllItems();
    $basetotal = 0;
    foreach ($items as $itemforprice) {
        $total = $this->processItem($itemforprice);
        $basetotal += $total;
    }

    $quote->setSubtotal($basetotal);
    $quote->setGrandTotal($basetotal);
    $quote->setBaseSubtotal($basetotal);
    $quote->setBaseGrandTotal($basetotal);
    $quote->setSubtotalWithDiscount($basetotal);
    $quote->setBaseSubtotalWithDiscount($basetotal);
    $quote->save();
}

private function processItem($itemforprice)
{
    $myprice = 100;
    $total = $myprice * $qty;

    $itemforprice->setCustomPrice($myprice);
    $itemforprice->setOriginalCustomPrice($myprice);
    $itemforprice->setBasePrice($myprice);
    $itemforprice->setPrice($myprice);

    $itemforprice->setRowTotal($total);
    $itemforprice->setBaseRowTotal($total);
    $itemforprice->setRowTotalInclTax($total);
    $itemforprice->setBaseRowTotalInclTax($total);

    $itemforprice->save();

    return $total;
}

$this->_eventManager->dispatch('sales_quote_item_qty_set_after', ['item' => $this]);