@Fabien Brugière The pricing for the speech services is based on the usage of the services per hour or no. of characters based on the features you are using for speech services. The details of which are available on the pricing page. Your usage can be estimated form the pricing calculator or you can lookup your usage from the cost analysis tab on the cost management blade of your subscription.
I think the estimate of €88 per month is probably the estimate from the calculator page based on your inputs for standard tier because the final billing is completely based on your usage and you can incur either more or less based on the features used. I hope this helps.