How much it cost to travel by underground from Frankfurt to Paris?

It depends where in Belgium to where in Germany.

As Belgium and Germany border one another, you could get a local train one stop across the border, or you could travel from Ostend to Frankfurt-an-der-Oder.

It also depends on when you travel, how far in advance you book, whether you travel on a normal or high-speed train and whether you travel first or second class.

Purely as an example. Single Cologne to Brussels, second class, for tomorrow, NOT using a High-Speed train is between €41.40 and €51.40.

These prices are from the German Railways website. See below for a link.