```python
from fastapi import FastAPI
import asyncio
import helpers  # noqa: F401 (the provided API)

app = FastAPI()


@app.get("/portfolio")
async def get_portfolio(symbols: str):
    """
    Fetch prices for the given comma-separated symbols.
    
    Returns:
        {"prices": {symbol: price, ...}}
    """
    # Parse symbols, handling any extra whitespace
    symbol_list = [s.strip() for s in symbols.split(",")]
    
    # Concurrently fetch prices using the async helper
    tasks = [helpers.afetch_price(sym) for sym in symbol_list]
    prices_list = await asyncio.gather(*tasks)
    
    # Map symbols to their fetched prices
    prices = dict(zip(symbol_list, prices_list))
    
    return {"prices": prices}
```