With the variable price you are allowing your customer to specify the amount. There is no option to do a minimum amount for that.
What you can do is put a text message below the subscription button (or in the product description) that clearly explains what is the minimum you are willing to accept. If anyone pays less then you can cancel the subscription and not deliver the service. If you explain it in the description of the product, then the visitors will pay within the range you are advertising.