I think the idea of making payments to yourself until you have saved enough to pay for having someone quilt for you is not a bad idea. As a longarm owner, I would be very hesitant to rent out my machine to a stranger. Kind of like letting a stranger drive your car (it nearly cost as much as my car). Shops that rent out their machines have to have extra insurance in case anything happens to the machine or the user because accidents do happen even to experienced quilters. That would be another reason someone would be hesitant to want to do this. Perhaps you can find someone who is new to longarming and would take your quilts to help them build up confidence. I did that when I was starting out.
Maybe you aren't really looking for solutions, just want to vent your frustration at not being able to afford to send out your quilts. We all sympathize. Quilting really is an expensive hobby.
Tall Chick's idea is a good one, too.