```python
from fastapi import FastAPI
from typing import List, Dict

import helpers  # noqa: F401  (the provided API)

app = FastAPI()


@app.get("/portfolio")
def portfolio(symbols: str):
    symbol_list = [s for s in symbols.split(",") if s]
    
    async def fetch_all():
        tasks = [helpers.afetch_price(symbol) for symbol in symbol_list]
        return await asyncio.gather(*tasks) if tasks else []

    results = asyncio.run(fetch_all()) if symbol_list else []
    prices = {symbol: price for symbol, price in zip(symbol_list, results)}
    
    return {"prices": prices}
```