From wherever you are, the cheapest way is almost certainly going to be flying to Bangkok then overland Bangkok-Cambodia using the Aranyaprathet-Poipet border crossing. There are alternative crossings, depending where your friend is/where you want to go in Cambodia. Read
this site and note the border visa scam on paying in baht--avoid this completely by getting your Cambodian visa in advance. Note you can fly to Phnom Penh but in high season (winter), that cheapie Air Asia flight is going to be booked heavily so reserve in advance if that's what you want.
The biggest determinant of cost is going to be getting to Bangkok, and that is determined by WHEN you fly. The following times are generally the most expensive for intercontinental tickets (the transpac part if flying from North America): July, August, mid-December to mid January, Chinese New Year (in 2010 Feb 14, so 2 weeks either side of that). Avoid these times if you can. I have found April-early June and late October-early December to have better air ticket pricing for just about any transpacific city-pair crossing, whether BKK, KUL, SIN, HKG, PVG, PEK, etc. If coming from Europe, this timing generally holds true as well. You will be lucky to find a ticket for $1000. If you think oil prices (jet fuel) will rise between now and time you go, buy ticket sooner rather than later. If in North America, start out with
www.thaifly.com