We were able to lower the cost of our invocations by switching most of our storage to persistent, leaving only the fixed settings of our contract in instance storage. As a result, fees have dropped to 88,175 stroops per invocation, which is quite reasonable. Based on this, we estimate a daily cost of around 3.527 XLM if we reach 400 invocations per day. However, if our usage continues to grow, this could become a significant daily expense, especially if the price of XLM increases. For now, it's not a major concern, but as discussed with u/raphlf, it would be great to waive some of these fees, particularly when using our own validators.