I was in Cambodia this time last year.
I found I spent more money in Cambodia than I did in Thailand.
Not sure how you are getting to Cambodia... if you are doing a land crossing, seriously think twice about it... or leave it for the end of your trip when going back to Thailand.
Taking the ferry from Thailand to Cambodia (entering at Sihanoukville) is a better way to come into Cambodia. Need more info? Message me on that... dont want to stray off the topic.
I spent a month there and took $1200US That would have been enough if I didnt have some money go missing (my own fault), or keep buying stuff for the homeless children.
You may need a steel heart for Cambodia, its such an amazing place, but so saddening at the same time, and I couldnt help but spend a good portion of my budget on anything and everything that they try and sell you.
Might be nice if you take a HUGE bag of balloons... might get you out of having to buy things ☆
Brush up on your capital cities of the world too.. im not joking, they will bribe you hahaha, and they know the answer to everything they ask you!
Oops off topic again, sorry...
Siem Reap our accomodation was $4 a night, with fan and cold shower (maybe less)... you honestly dont need a hot shower there. We hired motorbike drivers for a few days and I think we payed them $20 but we heard that there were cheaper guys around, but our guys took us out to Cambodian bars and stuff, so it was a cultural experience.
Phnom Penh the accomodation was cheaper, still just a fan and cold shower, maybe $3 a night? We stayed on the river.
You can hire your own motorbikes in Phnom Penh.
Sihanoukville was about $5 a night, hot shower, and fan bungalow. We hired our own motorbikes there for $2 a day.
Food is relatively cheap in Cambodia... just dont let yourselves get ripped off. Basically anything and everything they will try to sell you for $1 or $2 (they use $US there, no matter what they try to tell you at the dodgy exchange places)....
Barter, barter, barter....
Didnt go to the islands in Thailand, so im not sure how much you need for there, but northern thailand was really cheap.
Again, Bangkok, I stayed with a friend, so Im not sure the general price there either... I think it might pay to spend a little more there... some places I heard can be a rather grotty.
Overall if you take $1000 and dont go silly buying every little something that they try to sell you, you will have more than enough.