Please make sure that you change the prim limit to what you need. Since the price of the rental is based on the prim numbers, you can save L$ by using low-prim furniture. Also you can change the prim number at any time – adjusting the remaining rental time. To find out how many prims you are using check on here:
http://www.socionex.com/rental/primCount.jsp
If you use more prims, your rental price/prims will be adjusted. So please manage your own prims 🙂