That's ten days (drop-off is so many hours beyond nine 24-hour periods that you're legitimately being charged an extra day).
Try two five-day rentals, even if it means taking time out from your trip for the renter to return to the airport. A rental car week equals five to seven 24-hour periods.
See mousesavers.com for discount codes and advice.
See bnm.com and carrentalmomma.com.
Try the car rental codes at vacationoutlet.com.
What about Priceline or Hotwire? You're locked into the rental (well, not really - but you've paid for it, no refunds) and you don't find the rental agency name until after the transaction is completed.
If you're staying onsite, do you NEED a car? The whole time?