Is the additional amount requested for subscribing to the elevator, after an initial refusal to subscribe, considered usury or a legitimate increase, given that the increase is due to rising prices?

The additional amount is not usury (riba), because it is not a debt exchanged for a term. Rather, it is the purchase of a service or a share in the elevator, agreed upon by both parties. The increase results from rising prices, and the neighbors had previously stipulated that anyone who joins in the future would pay according to the value of the elevator at that time.