While Cambodia truly is still a cheap country you may find some of the hints here useful if your budget is really tight.
If you're interested in an alternative itinerary, here's the detailed description of our bicycle tour in Cambodia, complete with map and elevation profile:
part1: from Koh Khong Thai border to Kampot
part2: from Kampot to Phnom Penh
part3: from Phnom Penh to the Lao Border. Kampong Cham, Kratie and Stung Treng
camping in Cambodia
Prices are pretty even, ranging from 5USD to 12USD for a double room with fan and usually 5/7 dollars more for A/C. If you have any choice ask to see the room first, check the pillows, the mattress and, most important, the fan. Bargaining is worth a try, mostly works in tourist areas.
Since drinkable water is not the easiest nor the cheaper thing to get, if the place has a water dispenser may be worth a couple bucks more, if you have bottles to fill.
Especially in not touristic areas guesthouses are usually love-motel style, ground floor, which is nice if you have heavy baggage and bicycles.
Some rooms have water kettle but if there isn't any just ask the owner to provide some boiled water if you need it.
Many guesthouses, even the one you wouldn't say, have free WiFi.
Guesthouses are normally announced by signs, sometimes very small, keep your eyes wide open, if in a un-touristy area missing one could mean having no other one for a long while!
sleeping in Buddhist temples
Worst of all, at least in our experience, is being woke up one hour before dawn by the preparation for the morning prayer, if you are a heavy sleeper you may not notice, but no chance you'll oversleep the huge drum being played at dawn. So if you're up for a early rise and want to try the experience, well do it, at least once. But if a good night of sleep is what you're after my advice is to stick to guesthouses and resort to temples only if you have no other choice.
When we did it we left a 4USD donation (the elder monk asked for it), just two more dollars and we could had had a real bed, a shower, and sleep as long as we wanted, if only there was a guesthouse there...
couchsurfing and warmshower? or workaway, helpx and woofing?
eateries and restaurants
Western food can be found in every touristic town, usually in guesthouses, being a mix of fake Italian spaghetti and American style stuff such as hamburgers, tacos and ribs. Prices are not cheap, from 4 to 6USD for a burger that may not fulfill your needs.
Try the crabs in Kep, that is cheap and very tasty, and the Italian food stall in Kampot.
shops, markets and supermarkets
Markets are the right places to buy fruit, bergain hard. They also have som food stalls, better off in the morning when the food is freshly cooked, than in the afternoon, when it has been already some hours in the unforgiving heat.
If you just came here from a developed country beware of food poisoning, your stomach may not be equipped to handle the germs on the lady's hand. I got poisoned to in a market, even though I was coming from 8 months in SEA, and felt I could stand the bad hygene. Sometimes we resorted to instant noodles, but be careful, once we saw the lady of the shop taking the water for the noodles from a rusty barrel collecting rain from an asbestos roof.
Oh, I almost forgot the most himportant of the hints! Almost all Cambodian beers can give a prize, look under the tab of the can when opening it, if there's the symbol of a can you won one free beer! It's surprisingly common to win free beers, almost one on three or for cans is a lucky one!
baguette and bakeries
Bakeries are rare, better on sweets than salty pastry. However when we saw one we usually stock a few pieces. Not bad overall, considerng the average quality of Cambodian food.
English is almost completely unknown outside of touristic areas and google doesn't have a downloadable khmer language package for translate. Rely on gestures and simple phrasebook, order food by pointing at somebody's else dish, or just cope with whatever they might bring you, which would probably be a noodle soup anyway.
This being a generalisation, Cambodian people are mostly friendly but not very helpful, don't rely on their direction so much and don't expect them to help dragging your bike out of the mud.
When leaving the main road hunting for alternative routes the whole story gets a 180 turn. It's here that the real Cambodia reveals itself, the slow rhythms of a country which somehow still lives in the middle ages. Hand-pulled wooden looms, ritual horse carts, run down temples and people wearing traditional clothes. This all comes at a price, which is basically clouds of red dust in dry season and pools of mud during the wet one. While dust may not be nice but still bearable with a bit of cover up, mud can make some road completely unrideable, a hell on earth sometimes. Think twice when trying one of those, check if you'll have any chance to get back to the pavement and avoid the risk if there was a recent downpour longer than a few hours, try asking the locals but don't trust them 100%, sometimes they will say it's ok when really it is not, maybe ask more people.
Respect the basic rules: money on you, passport on you, do not leave nothing valuable or important in your backpack (or whatever your baggage is) if this is loaded in the trunk of a bus or taxi, don't pay stuff in advance, trust your instinct, don't make rushed choices, don't be drunk or high if you have valuable stuff with you.