import re


def convert_price(line):
    """Convert line to price (float) or raise ValueError"""
    # line might be '£2.90' or something else
    matches = re.findall("(\d+\.\d+)", line)
    if not matches:
        raise ValueError()
    price = float(matches[0])
    assert price >= 0, "We expect positive prices over £0.0"
    assert price < 100, "We don't expect a charge over £100"
    return price