The cheapest way is to take a bus from Kao San Road in Bangkok to Siem Riep. Be warned that some of these buses are also SCAMS, meaning it’s a dusty, bumpy 9 hour ride that attempts to trick you into paying more for Visas at border. They drive slower than needed to get you there late, then upsell a hotel. It’s an $60 “experience”, but after doing it the $300 plane ride seems worth it.

If you’re going to take the bus, ensure you pay up front for the visa and pack a headlight, facemask, pillow, and reading material for this bus.

The cheapest way to get there would be overland, by public bus to Aranyaprathet, and changing buses at the border crossing into Poipet. But it’s not a particularly fun trip: it takes the better part of a full day, Poipet is the ugliest town in Cambodia, and there are a lot of ripoff artists at both the border and among “VIP” transit companies that ply their trade at the cheap guesthouses.

If you insist on going by bus, assuming you’re in Bangkok, DON’T take one of the buses advertised on Khao San Road—go to the Ekkamai bus station and book a regular bus. You’ll probably have to change in Chachoengsao.

A more expensive, but still cheap, and hundreds of times more pleasant option, is to fly from BKK-Phnom Penh on Air Asia. Takes around an hour, and costs $100 or less. Totally, totally worth it.

the bus is definitely the cheapest, but 9 hours is generous. it took me 14 hours. also, when you cross into cambodia, they change buses to a school bus with no air conditioning. the road is not paved and reading is impossible. definitely get your visa in advance. you can do it online if you have enough time. i didn’t stay at the guest house they dropped us off at. it was close enough to walk into the main part of siem reap and find something there. i recommend doing that as it’s cheaper and better to be right where all the bars and restaurants are. the plus side of taking the bus…i met a bunch of other really great people!

I agree with Daniel, you’d be better off flying, if you can afford it and especially if you’ve not travelled around Asia much. There’s a good reason why Poipet is sometimes referred to as the toilet of South East Asia….
Another thing is that road from Poipet to Siem Ream is in really bad condition and especially during the rainy season it’s a very unpleasant trip, the buses can get stuck in the mud…
I usually enjoy road trips and they’re fun in South East Asia, but this is one (and probably the only) route I’d rather not go by road!
If you do decide to do it, read about it online a bit (there are lots of good guides, think Asia tiger or something like that is one?) and read about Poipet and the nuisances/dangers there so you’re well prepared and know what to do/where to go.