My experience has been shop all the providers. Penske, UHaul, Hertz, etc. Both online and by phone. Get the lowest price than contact all of them again with your best quote. Then go back to the one who gave you the best quote originally and they will usually beat the others again. Penske usually wins.
Helpful tip for one-way rentals. Try providing alternative drop off cities. For example: It was hundreds less to drop off in Washington D.C. than in North Carolina. Like $500+ less. After getting to NC from Denver and anticipating having to go to D.C to drop off, the local operation told me they can't disallow you to drop off the vehicle anywhere. I left it in NC and there was no extra charge.
KTM 950 Super Enduro, KTM 950 Adventure, KTM 950 Supermoto
Beta 520 RS, BMW R1200 GSA, Rickman Cafe Racer