I just got back from Lombok and had a great time trekking up Mt Rinjani and chilling at the Gili Islands!
If you are planning to do the trek, you might want to consider doing it other than January to March as the Mt Rinjani National Park is closed due to the rainy season.
Do consider engaging the service of John's Adventure as I think they are really good and professional.
Visit their website at: http://www.rinjanimaster.com/
Read about the reviews on Tripadvisor: http://www.tripadvisor.com.sg/ShowUserReviews-g297733-d386938-r148250930-Mount_Rinjani-Lombok_West_Nusa_Tenggara.html
So my group of 4 paid for the 3 Day 2 Night, Deluxe Plus Package for USD$235 + tips each. A guide and 4 porters accompanied us throughout the whole trekking. And we were taken care of very well.