cheap option for powering a bi-pap/cpap machine

I use the Resmed S9 with humidifier. When it first came out I had to use a cheap inverter. After about a year later they did come out with a DC cord that plugs in to lighter socket. Works great. Expensive but worth it IMO. Using 2 trojan t-105 6 volt batts and have never had a problem even when using humidifier.