Cost/savings to replace 73 year old furnace?

Had 2 companies come take a look they said the bb is working and fine so we don’t have to replace if we don’t want. It was made in 1950 and installed 1951- 73 years old! but they didn’t test for efficiency. Our bill was hovering around $300 a month so didn’t bother to think about it since we moved in back in May, but December was $900. Cost to replace is about $15k plus $2-3k for the asbestos remediation. Worth it? lol can’t believe I’m even asking. I didn’t get a quote for insulation yet but I bet it’s like $12k it’s a large house. Currently the house has 0 insulation.

$30k is such a big investment to save a few hundred a month on the utility bill