要计算AWS GP2存储的收费，可以使用AWS的价格计算器API来获取价格信息。下面是一个使用Python的示例代码：

import requests

def calculate_storage_cost(size_gb):
    url = "https://pricing.us-east-1.amazonaws.com/offers/v1.0/aws/AmazonRDS/current/index.json"
    response = requests.get(url)
    data = response.json()

    # 找到GP2存储的相关信息
    product = None
    for offer_code, offer_data in data['offers'].items():
        if 'AmazonRDS' in offer_data['product']['productFamily'] and 'Storage' in offer_data['product']['attributes']['usagetype']:
            product = offer_data['product']
            break

    if product:
        # 找到GP2存储的价格信息
        price_dimensions = product['priceDimensions']
        for price_dimension in price_dimensions.values():
            price_per_gb = float(price_dimension['pricePerUnit']['USD'])
            break
        else:
            return None

        # 计算存储费用
        storage_cost = price_per_gb * size_gb
        return storage_cost

    return None

# 示例调用
storage_size_gb = 100
cost = calculate_storage_cost(storage_size_gb)
if cost:
    print(f"The cost of {storage_size_gb} GB of GP2 storage is ${cost:.2f}")
else:
    print("Failed to calculate storage cost")

这段代码使用了AWS Pricing API来获取GP2存储的价格信息。首先，它发送一个GET请求来获取价格信息的JSON数据。然后，它遍历JSON数据，找到GP2存储的相关信息，并计算出存储费用。

请注意，此示例代码仅用于演示目的，实际使用时可能需要根据具体情况进行修改。