APL producing HTML, score 10896.1

(Score is \$\text{len}(f_\text{main})×(\sum_{i=1}^{100}{\text{len}(f_\text{main}(p_i))\over100})^{1.5}\$.)

⍞

This prompts for a line of input and outputs it verbatim (example). Since none of the paragraphs contain < or & or multiple consecutive spaces, then the output is valid HTML that will produce itself (a trivial quine).

A naive example in Python that produces Python — (168 bytes; score is 1667263.6473830533)

import zlib,base64;print(f"import zlib,base64;print(zlib.decompress(base64.b85decode('{base64.b85encode(zlib.compress(input().encode())).decode()}')).decode(),end='')")

Testing script (not part of challenge)

import subprocess
import io
import sys

with open("golf.py", "r") as f:
    size_of_program = len(f.read())

def run_program(cmd, input_str=""):
    result = subprocess.run(
        cmd,
        input=input_str.encode(),
        stdout=subprocess.PIPE,
        stderr=subprocess.PIPE,
        shell=True
    )
    return result.stdout.decode()

def execute_python_with_stdout_capture(python_code, input_str=""):
    output_buffer = io.StringIO()
    sys.stdout = output_buffer
    exec(python_code, {'input': lambda: input_str})
    sys.stdout = sys.__stdout__
    captured_output = output_buffer.getvalue()
    output_buffer.close()
    return captured_output


if __name__ == "__main__":
    with open("para.txt", "r") as f:
        file_content = f.readlines()

    total = 0
    for line in file_content:
        line = line.strip()
        output = run_program("python3 golf.py", line)
        code_output = execute_python_with_stdout_capture(output)
        if code_output!=line:
            print(f"{code_output}\n{line}")
            raise ValueError("Mismatch! Your program did not output the expected result and does not qualify!")
        total+=len(output)
    score = size_of_program*((total/100)**1.5)

    output = run_program("python3 golf.py")
    print("Your program works! Score:", score)
```

Python 3.8 (pre-release), score 465379.4785138254 (39 bytes, Python producing Python)

If it ain't broke, don't fix it.

print(f"print('''{input()}''',end='')")

Testing script (not part of challenge, only tiny changes from OP's)

import subprocess
import io
import sys

with open("golf.py", "r") as f:
    size_of_program = len(f.read())

def run_program(cmd, input_str=""):
    result = subprocess.run(
        cmd,
        input=input_str.encode(),
        stdout=subprocess.PIPE,
        stderr=subprocess.PIPE,
        shell=True
    )
    return result.stdout.decode()

def execute_python_with_stdout_capture(python_code, input_str=""):
    output_buffer = io.StringIO()
    sys.stdout = output_buffer
    exec(python_code, {'input': lambda: input_str})
    sys.stdout = sys.__stdout__
    captured_output = output_buffer.getvalue()
    output_buffer.close()
    return captured_output


if __name__ == "__main__":
    with open("para.txt", "r", encoding="utf-8") as f:
        file_content = f.readlines()

    total = 0
    for line in file_content:
        line = line.strip()
        output = run_program("python golf.py", line)
        code_output = execute_python_with_stdout_capture(output)
        if code_output!=line:
            print(f"{code_output}\n{line}")
            raise ValueError("Mismatch! Your program did not output the expected result and does not qualify!")
        total+=len(output)
    score = size_of_program*((total/100)**1.5)

    output = run_program("python golf.py")
    print("Your program works! Score:", score)
```