How much would it cost to subscribe to a site with music without copyrights? I found one service that asks from $30 a month, this is for the cheapest plan. The most expensive one is more than $70 a month.

For that cost you have to have the music writers write the music themselves to your order, not so you can for that cost download a ready-made track that has already been used at least a hundred times.

I agree with your expression, but unfortunately I can't find another service. But I would be very glad if you could tell me where to find another option? I was told to try to find one in social networks.

It's not a bad option to look in social networks, but it's not very workable. As novice musicians will not make you a quality track, and popular ones will ask a huge amount for their services. Try to use copyright free music service. Some of my friends, including me, always download music from this service for their projects and commercials. As far as I know big companies also take music from this service for their videos.